效果

wxml

<button type="primary" form-type='submit'>登录</button>

wxss

添加如下样式：

button[type="primary"] {
    display:flex;
    align-items: center;
    justify-content: center;
}

所有样式如下：

button[type="primary"] {
    display:flex;
    align-items: center;
    justify-content: center;
    height: 40px;
    line-height: 40px;
    font-size: 36rpx;
    text-align: center;
    background-color: #007AFF !important;
    margin-top: 20px;
    margin-bottom: 20px;
    color: #FFFFFF;
    border-radius: 10px;
}

 效果如下