const cssString = 'font-size:20px;width:158px;line-height:24px;font-family:MinweiQagan;';

const regex = /(?<=^|;)\s*([^:]+)\s*:\s*([^;]+)\s*/g, styleObject = {};

cssString.replace(regex, (m,p,v) => {styleObject[p] = v});

console.log(styleObject);