感谢维安雨轩提供思路

修改主题设置

在后台的主题设置中打开展开导航菜单

修改header.php

打开header.php,找到<header class="site-header no-select" role="banner">，将其替换为

<?php if (is_front_page()){ ?>
<header class="site-header no-select is-homepage" role="banner">
<?php }else{ ?>
<header class="site-header no-select" role="banner">
<?php } ?>

然后搜索</div><!-- .site-branding -->

在下一行添加

<div class="lower-cantiner">

并将

<div class="lower"><?php if(!akina_option('shownav')){ ?>
	<div id="show-nav" class="showNav">
		<div class="line line1"></div>
		<div class="line line2"></div>
		<div class="line line3"></div>
	</div><?php } ?>
    <nav><?php wp_nav_menu( array( 'depth' => 2, 'theme_location' => 'primary', 'container' => false ) ); ?></nav><!-- #site-navigation -->
</div>

修改为

<div class="lower"><?php if(!akina_option('shownav')){ ?>
	<div id="show-nav" class="showNav">
        <div class="line line1"></div>
		<div class="line line2"></div>
		<div class="line line3"></div>
	</div><?php } ?>
        <nav class="mobile-fit-control hide"><?php wp_nav_menu( array( 'depth' => 2, 'theme_location' => 'primary', 'container' => false ) ); ?></nav><!-- #site-navigation -->
</div>

然后移到

<?php header_user_menu(); if(akina_option('top_search') == 'yes') { ?>
<div class="searchbox"><i class="iconfont js-toggle-search iconsearch icon-search"></i></div>
<?php } ?>

的上方

修改style.css

添加代码

在style.css文件尾部添加下列代码

.site-header.is-homepage .lower-cantiner {
    display: none;
}
.site-header.is-homepage.yya .lower-cantiner {
    display: block;
}
.site-header:hover.is-homepage .lower-cantiner {
    display: block;
}
.site-top .lower-cantiner {
    position: absolute;
    left: 50%;
    min-width: 758.4px;
    pointer-events: none;
}
#show-nav.mobile-fit {
    position: fixed !important;
    right: 120px;
    opacity: 0;
    pointer-events: none;
}

修改代码

搜索 .site-top .lower { 并将其修改为

.site-top .lower {
    position: relative;
    display: inline-block;
    margin: 15px 0 0 10px;
    font-size: 16px;
    left: -50%;
    pointer-events: auto !important;
}

搜索 @media(max-width:1200px) { 并将其修改为

@media(max-width:1200px) {
    .site-top .lower nav{
        position: absolute;
        float: right;
        animation: searchbox .2s;
        min-width: 860px;
        z-index: -1
    }
    .site-top .lower nav.navbar {
        right: calc(-50px - 100%);
    }
    .site-top .lower nav.navbar ul {
        background: #fff;
    }
}