要将flex项目在不同屏幕上设置为相等的宽度，请使用flex-*-fill类。
对于小宽度的屏幕，请使用flex-sm-fill-
<div class="p-2 flex-sm-fill bg-info"> A </div>
对于大宽度的屏幕，请使用flex-lg-fill-
<div class="p-2 flex-lg-fill bg-info"> A </div>
以下是为不同屏幕上的伸缩项目提供euqla宽度的示例-
<!DOCTYPE html> <html lang="en"> <head> <title>Bootstrap Example</title> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1"> <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css"> <script src="https://cdn.staticfile.org/jquery/3.3.1/jquery.min.js"></script> <script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script> </head> <body> <div class="container mt-3"> <h3>Flex (Small Width)</h3> <div class="d-flex mb-3 bg-primary"> <div class="p-2 flex-sm-fill bg-info">A</div> <div class="p-2 flex-sm-fill bg-warning">B</div> <div class="p-2 flex-sm-fill bg-primary">C</div> </div> <h3>Flex (Medium Width)</h3> <div class="d-flex mb-3 bg-primary"> <div class="p-2 flex-md-fill bg-info">A</div> <div class="p-2 flex-md-fill bg-warning">B</div> <div class="p-2 flex-md-fill bg-primary">C</div> </div> <h3>Flex (Large Width)</h3> <div class="d-flex mb-3 bg-primary"> <div class="p-2 flex-lg-fill bg-info">A</div> <div class="p-2 flex-lg-fill bg-warning">B</div> <div class="p-2 flex-lg-fill bg-primary">C</div> </div> </div> </body> </html>