1、实现自定义Dialog 布局 dialog_main_sign.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:gravity="center"
   android:orientation="vertical">


   <ImageView
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:scaleType="fitXY"
       android:src="@drawable/ic_main_sign_pop" />

   <ImageView
       android:id="@+id/img_sing_x"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:layout_marginTop="@dimen/dp_20"
       android:src="@drawable/ic_main_sign_x" />

</LinearLayout>

2、代码

    private ImageView img_sing_x;

    public void showDialog() {
        AlertDialog.Builder builder = new AlertDialog.Builder(me.getContext());
        final AlertDialog dialog = builder.create();
        View dialogView = View.inflate(this.getContext(), R.layout.dialog_main_sign, null);
        dialog.setView(dialogView, 0, 0, 0, 0); // 去边距，兼容2.X
        dialog.getWindow().setBackgroundDrawableResource(android.R.color.transparent);
        img_sing_x = (ImageView) dialogView.findViewById(R.id.img_sing_x); // guanbi

        dialogView.findViewById(R.id.img_sing_x).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                dialog.dismiss();
            }
        });

        dialog.show();
    }

注意： dialog.getWindow().setBackgroundDrawableResource(android.R.color.transparent);
这才是实现背景透明的重点