<div class="wrap" id="img">
    <img class="prew" src="https://tva1.sinaimg.cn/large/008i3skNgy1gubr2sbyqdj60xa0m6tey02.jpg">
</div>

body{
    margin: 0;
    height: 100vh;
    display: grid;
    place-content: center;
}
.prew{
    display: block;
    width: 800px;
}
.wrap{
    position: relative;
  overflow:hidden;
}
.wrap::after{
    content: '';
    position: absolute;
  width: 100px;
  height: 100px;
  border-radius: 50%;
  left: calc(var(--x, .5) * 100%);
  top: calc(var(--y, .5) * 100%);
  transform: translate(-50%,-50%);
  box-shadow: 0 0 0 999vw rgba(0, 0, 0, .5); /*足够大的投影*/
}

img.addEventListener('mousemove', (ev) => {
    img.style.setProperty('--x', ev.offsetX / ev.target.offsetWidth);
    img.style.setProperty('--y', ev.offsetY / ev.target.offsetHeight);
})