方案一:

function remSize(){
    // 获取设备宽度:
    var deviceWidth = document.documentElement.clientWidth || window.innerWidth
    if(deviceWidth >=750){
        deviceWidth=750
    }
    if(deviceWidth<=320){
        deviceWidth = 320
    }
    // 设置根目录的字体大小:
    document.documentElement.style.fontSize = (deviceWidth/7.5) +'px'
    document.body.style.fontSize= 0.16 + 'rem'    
}
remSize()
window.onresize=function(){
    remSize()
}

方案二

 let active_w = 375;
 let active_s = 20;
 let new_w = document.documentElement.clientWidth
 document.documentElement.style.fontSize = (active_s * new_w / active_w)+'px';
window.onresize=function(){
       let new_w = document.documentElement.clientWidth
       document.documentElement.style.fontSize = (active_s * new_w / active_w)+'px'
        }

方案三

(function(win) {
        var docEl = win.document.documentElement;
        var time;
        function refreshRem() {
        var width = docEl.getBoundingClientRect().width;
        if (width > 768) { // 最大宽度
                        width = 768;
                }
        var rem = width / 375*100;
        docEl.style.fontSize = rem + 'px';
                ///rem用font-size:50px来进行换算
        }
    win.addEventListener('resize', function() {
                clearTimeout(time);
                time = setTimeout(refreshRem, 1);
        }, false);
        win.addEventListener('pageshow', function(e) {
                if (e.persisted) {
                        clearTimeout(time);
                        time = setTimeout(refreshRem, 1);
                }
        }, false);
        refreshRem();
})(window);