前提：已經完成了node js 的安裝。
步驟：
按照網上的命令開啟cmd直接執行以下命令：

 npm install -g @angular/cli

由於node-sass包被牆，可能會安裝報錯，此時執行以下語句：

npm install cnpm -g --registry=https://registry.npm.taobao.org

安裝淘寶cnpm時，系統報錯：

npm WARN deprecated [email protected]: If using 2.x branch, please upgrade to at least 2.1.6 to avoid a serious bug with socket data flow and an import issue introduced in 2.1.0
npm WARN tarball tarball data for 
 
[email protected]^2.18.1 (sha1-PCV/mDn8DpP/UxSWMiOeuQeD/2Y=) seems to be corrupted. Trying one more time.
npm ERR! path C:\Users\zhangjixun01\AppData\Roaming\npm\node_modules\.staging\moment-78cea585\min\locales.js
npm ERR! code EPERM
npm ERR! errno -4048
npm ERR! syscall unlink
npm ERR! Error: EPERM: operation not permitted, unlink 'C:\Users
 
\zhangjixun01\AppData\Roaming\npm\node_modules\.staging\moment-78cea585\min\locales.js'
npm ERR!  { [Error: EPERM: operation not permitted, unlink 'C:\Users\zhangjixun01\AppData\Roaming\npm\node_modules\.staging\moment-78cea585\min\locales.js']
npm ERR!   cause:
npm ERR!    { Error: EPERM: operation not permitted, unlink 'C:\Users
 
\zhangjixun01\AppData\Roaming\npm\node_modules\.staging\moment-78cea585\min\locales.js'
npm ERR!      errno: -4048,
npm ERR!      code: 'EPERM',
npm ERR!      syscall: 'unlink',
npm ERR!      path:
npm ERR!       'C:\\Users\\zhangjixun01\\AppData\\Roaming\\npm\\node_modules\\.staging\\moment-78cea585\\min\\locales.js' },
npm ERR!   stack:
npm ERR!    'Error: EPERM: operation not permitted, unlink \'C:\\Users\\zhangjixun01\\AppData\\Roaming\\npm\\node_modules\\.staging\\moment-78cea585\\min\\locales.js\'',
npm ERR!   errno: -4048,
npm ERR!   code: 'EPERM',
npm ERR!   syscall: 'unlink',
npm ERR!   path:
npm ERR!    'C:\\Users\\zhangjixun01\\AppData\\Roaming\\npm\\node_modules\\.staging\\moment-78cea585\\min\\locales.js',
npm ERR!   parent: 'cnpm' }
npm ERR!
npm ERR! The operation was rejected by your operating system.
npm ERR! It's possible that the file was already in use (by a text editor or antivirus),
npm ERR! or that you lack permissions to access it.
npm ERR!
npm ERR! If you believe this might be a permissions issue, please double-check the
npm ERR! permissions of the file and its containing directories, or try running
npm ERR! the command again as root/Administrator (though this is not recommended).

npm ERR! A complete log of this run can be found in:
npm ERR!     C:\Users\zhangjixun01\AppData\Roaming\npm-cache\_logs\2018-08-06T07_00_55_009Z-debug.log

說是許可權問題，但我許可權都放開了試了一遍也不行，又換了以下方法安裝：
1.安裝node-sass包：

set SASS_BINARY_SITE=https://npm.taobao.org/mirrors/node-sass/

npm install node-sass

2.然後再直接安裝angular：

npm install -g @angular/cli

此時成功！執行ng -v命令後：

>ng -v

     _                      _                 ____ _     ___
    / \   _ __   __ _ _   _| | __ _ _ __     / ___| |   |_ _|
   / △ \ | '_ \ / _` | | | | |/ _` | '__|   | |   | |    | |
  / ___ \| | | | (_| | |_| | | (_| | |      | |___| |___ | |
 /_/   \_\_| |_|\__, |\__,_|_|\__,_|_|       \____|_____|___|
                |___/


Angular CLI: 6.1.2
Node: 10.8.0
OS: win32 x64
Angular:
...

Package                      Version
------------------------------------------------------
@angular-devkit/architect    0.7.2
@angular-devkit/core         0.7.2
@angular-devkit/schematics   0.7.2
@schematics/angular          0.7.2
@schematics/update           0.7.2
rxjs                         6.2.2
typescript                   2.7.2

可以繼續接下來的建立新專案了。
（樓主小白，請大家多多指教，筆芯）

更新：翻了外網，發現有老外指出是AppData\Roaming下的兩個包
npm
npm-cache的問題，刪掉即可。