安装自定义用的插件

npm i -D @angular-builders/custom-webpack @angular-builders/dev-server postcss-px-to-viewport

检查一下@angular-builders的主版本号（7.x.x）是不是和我们的@angular/cli主版本一致，这里用angular7来示范。
修改angular.json文件，全局搜索

@angular-devkit/build-angular:browser

替换为

@angular-builders/custom-webpack:browser

在@angular-builders/custom-webpack:browser同级的options选项里添加

 "customWebpackConfig": {
              "path": "./webpack.config.js"
           },

全局搜索

@angular-devkit/build-angular:dev-server

替换为

@angular-builders/dev-server:generic

在angular.json同级目录下新增webpack.config.js，内容如下

var pxtoviewport = require('postcss-px-to-viewport');

function regexEquals(firstValue,secondValue){
    return firstValue+'' == secondValue +'';
}
const addPostCssPlugins = [//自定义的postcss插件
    pxtoviewport({
        viewportWidth: 750,
        viewportHeight: 1334,
        unitPrecision: 5,
        viewportUnit: 'vw',
        selectorBlackList: [],
        minPixelValue: 1,
        mediaQuery: false
    })
]
module.exports = (config, options) => {
//   console.log(config.module.rules);
  let rules = config.module.rules;
  let styleRules = rules.filter(rule => {
      let test = rule.test;
      return regexEquals(test,/\.css$/)|| regexEquals(test,/\.scss$|\.sass$/)|| regexEquals(test,/\.less$/)|| regexEquals(test,/\.styl$/);
  });
  console.log(styleRules)
  styleRules.forEach(rule => {
    //   console.log(rule);
     let currentPostCssLoader = rule.use.find(loader => loader.loader==='postcss-loader');
     let getPluginsFn = currentPostCssLoader.options.plugins;
     let getPluginsAddedFn = (loader)=>{
         return [...getPluginsFn(loader),...addPostCssPlugins];
     }
     currentPostCssLoader.options.plugins = getPluginsAddedFn;
    //  console.log(typeof currentPostCssLoader.options.plugins)
    //  console.log(currentPostCssLoader.options.plugins())
    //  currentPostCssLoader.options.plugins.push(...addPostCssPlugins);
  })
  return config;
};

然后，运行npm start,运行ng build --prod试验下效果，你会看到我们自定义的postcss插件已经起作用了。
附录：https://github.com/angular/angular-cli/blob/master/packages/angular_devkit/build_angular/src/angular-cli-files/models/webpack-configs/styles.ts