解决方法:自定义快捷键

我是缩小快捷键失效，所以将放大和缩小快捷键都设置了下.
文件->偏好设置-> 最底下有个高级设置，选择 打开高级设置 ，点击之后看到两个json格式的文件，打开第二个（conf.user.json）,用记事本(sublime，notepad++都可以)打开—>第17行“keyBinding”,

/** For advanced users. */
{
  "defaultFontFamily": {
    "standard": null, //String - Defaults to "Times New Roman".
    "serif": null, // String - Defaults to "Times New Roman".
    "sansSerif": null, // String - Defaults to "Arial".
    "monospace": null // String - Defaults to "Courier New".
  },
  "autoHideMenuBar": false, //Boolean - Auto hide the menu bar unless the `Alt` key is pressed. Default is false.

  // Array - Search Service user can access from context menu after a range of text is selected. Each item is formatted as [caption, url]
  "searchService": [
    ["Search with Google", "https://google.com/search?q=%s"]
  ],

  // Custom key binding, which will override the default ones.
  "keyBinding": {
    // for example: 
    // "Always on Top": "Ctrl+Shift+P"
    "放大": "Ctrl + Shift + j",
    "缩小": "Ctrl + Shift + k"
  },

  "monocolorEmoji": false, //default false. Only work for Windows
  "autoSaveTimer" : 3, // Deprecidated, Typora will do auto save automatically. default 3 minutes
  "maxFetchCountOnFileList": 500,
  "flags": [] // default [], append Chrome launch flags, e.g: [["disable-gpu"], ["host-rules", "MAP * 127.0.0.1"]]
}

我是设置的
“放大”: “Ctrl + Shift + j”,
“缩小”: “Ctrl + Shift + k”，可以根据自己需求来更改

之后再重启Typora,就可以用了