在aligned环境里，公式是默认右对齐的，如何使其居中对齐？

请注意markdown的围栏代码块的样式应为三个反引号`!!!

可以将aligned替换为gathered,此外在嵌套环境的外层只需用equation即可。

由于你的纸似乎有点小，用\zihao{5}在数学模式下调整字体大小不合理。我下面的MWE修改了纸张大小为a3paper.

\documentclass[12pt]{ctexart}
\usepackage[a3paper,top=2.5cm, bottom=2.5cm, left=2.5cm, right=2.5cm]{geometry}
\usepackage{amsmath}

\begin{document}
\begin{equation}
s.t.=\left\{
    \begin{gathered}
        \varDelta _k^{(p)} = \sum_{i=1}^{54} \sum_{j=1}^{2} c_{ijk}x_{ijk}^{(p)}\\
        \varPhi _k^{(p)} = \sum_{j=1}^{2} w_{jk}\min\left\{\sum_{i=1}^{64}u_{ijk}x_{ijk}^{(p)},S_{jk}\right\}\\
        \varTheta _k^{(p)} = \lambda \sum_{j=1}^{2} w_{jk}\max\left\{\sum_{i=1}^{54}u_{ijk}x_{ijk}^{(p)} - S_{jk}, 0\right\}\\
        x_{i2k}^{(p)}=0 \quad k=1,2,\cdots,16\\
        x_{ijk}^{(p)}x_{i2k}^{(p)} = 0 \quad k\in R\\
        x_{i2k}^{(p)} x_{i1k}^{(p+1)}=0 \quad k\in R\\
        x_{ijk}^{(p)}x_{ijk}^{(p+1)}=0 \quad k\in (G\cup \{k=16\})\\
        \sum_{k\in G}^{}x_{ijk}^{(p)}=0 \quad i\notin D\\
        x_{i1(16)}^{(p)}\left(\sum_{j=1}^{2}\sum_{k\in R}^{} x_{ijk}^{(p)}\right) = 0 \quad i\in L\\
        \sum_{k\in R/R_1}^{} x_{i1k}^{(p)} + \sum_{k\in R_1}^{} x_{i2k}^{(p)} = 0, \quad i\in L\\
        \sum_{k\in R/R_1}^{} x_{i1k}^{(p)} + \sum_{k\notin F}^{} x_{i2k}^{(p)} = 0, \quad i\in N\\
        \sum_{i\in N,k\in F}^{} x_{ijk}^{(p)} + \sum_{j=1}^{2}\sum_{i\notin N,k\in F}^{} x_{ijk}^{(p)} = 0\\
        \sum_{i\in L,k\in R/R_1}^{} x_{j1k}^{(p)} + \sum_{j=1}^{2} \sum_{i\notin L,k\in R/R_1}^{} x_{ijk}^{(p)} = 0\\
        \sum_{k\notin G}^{} x_{ijk}^{(p)} = 0 \quad i\in D;x_{i1(16)}^{(p)} = 0 \quad i\notin L\\
        x_{ijk}^{(p)} \ge \frac{1}{2}A_i\text{sgn}(x_{ijk}^{(p)})\\
        \sum_{t=0}^{2}\sum_{k\in M}^{} x_{ijk}^{(p+t)} >0\\
        \sum_{k\in R/R_1}^{} \text{sgn}(x_{i2k}^{(p)}) \leq 1 \quad i\in L\\
        \sum_{k=1}^{n} x_{ijk}^{(p)} \leq A_i;\sum_{k}^{} \text{sgn}(x_{ijk}^{(p)}) \leq 1, \quad i\in D\\
        x_{ijk}^{(p)} \ge 0; \quad x_{ijk}^{(p)}=0,i\in Z,k\in R/R_1\\
        M = \{k| k=1,2,\cdots,5,17,18,19\}\\
        D = \{i| i=1,2,\cdots,26\};L = \{i| i=27,28,\cdots,34\}\\
        N = \{i| i=35,36,\cdots,50\};G = \{k| k=1,2,\cdots,15\}\\
        R_1=R/\{k=35,36,37\};Z = \{i| i=51,52,\cdots,54\}\\
        R = \{k| k=17,18,\cdots,37\};F = \{k| k=38,39,40,41\}\\
    \end{gathered}
\right.
\end{equation}
\end{document}

参考阅读（lshort-zh-cn链接）：

• 4.4节内容

• 4.7节内容