解决方法：
去 node_modules/react-navigation/src/views/Header/Header.js中的 title 属性一栏里

_renderTitle(props, options) {
    const { layoutPreset, transitionPreset } = this.props;
    let style = [
      { justifyContent: layoutPreset === 'center' ? 'center' : 'flex-start' },
      getTitleOffsets(
        layoutPreset,
        false,
        options.hasLeftComponent,
        options.hasRightComponent
      ),
      options.headerTitleContainerStyle,
    ];

  return this._renderSubView(
      { ...props, style },
      'title',
      this._renderTitleComponent,
      transitionPreset === 'uikit'
        ? this.props.titleFromLeftInterpolator
        : this.props.titleInterpolator
    );
  }

修改为：

  _renderTitle(props, options) {
    const { layoutPreset, transitionPreset } = this.props;
   // 主要把 flex-start 改成  center
    let style = [
      { justifyContent: layoutPreset === 'center' ? 'center' : 'center' },
      getTitleOffsets(
        layoutPreset,
        false,
        options.hasLeftComponent,
        options.hasRightComponent
      ),
      options.headerTitleContainerStyle,
    ];

    return this._renderSubView(
      { ...props, style },
      'title',
      this._renderTitleComponent,
      transitionPreset === 'uikit'
        ? this.props.titleFromLeftInterpolator
        : this.props.titleInterpolator
    );
  }