在这种情况下，你必须添加一个自定义指令：

.directive('openOnFocus', function () {

  function compile(tElement) {
    tElement.find('input').attr('ng-focus', 'ctrl.onFocus($event)');

    return function (scope, element, attrs, datePicker) {
      var focused = false;

      datePicker.onFocus = (event) => {
        focused = !focused;

        if(focused) {
          datePicker.openCalendarPane(event);
        }
      };
    };
  }

  return {
    compile: compile,
    priority: -1,
    require: 'mdDatepicker',
    restrict: 'A'
  };
});

然后：
<md-datepicker open-on-focus></md-datepicker>
摘自Github评论：
https://github.com/angular/material/issues/4650#issuecomment-191930096