将hexidecimal color-codes替换为rgba color-codes：

div {
width: 150px;
height: 150px;
background-image: linear-gradient(to left, rgba(0,67,116,0.6) -26.48%, rgba(0,51,102,0.6) 73.52%);
}

<div>
</div>

使用RGBa值：

linear-gradient(to left, rgba(0,73,122,0.6) 26.48%, rgba(0,51,102,0.6) 73.52%);

试试这个：

background-image:linear-gradient(rgba(0,0,0,0.9),rgba(0,0,0,0.9)), url('/image/r.jpg');

若要设置背景中图片本身的透明度而不应用颜色，可以使用background mask属性

background-image: url('path/to/your/image.jpg');
-webkit-mask-image: linear-gradient(to bottom, transparent 40%, black 55%);
mask-image: linear-gradient(to bottom, transparent 40%, black 75%);

您可以在这里阅读更多https://www.digitalocean.com/community/tutorials/css-masking-with-mask-image