Thank you for your reply! And I had figured out why it didn't work. You are right. My frame buffer was empty. To solve this problem, I just put the getPixels() method in the onDrawFrame like this:
```
public int[] tmpPixels;// Global variance
public void onDrawFrame(GL10 gl) {
            tmpPixels = fb.getPixels();
            fb.clear(back);

            world.renderScene(fb);
            world.draw(fb);
            fb.display();
}
```
Then I convert the int array (tmpPixels) to bitmap and save it in my dispatchKeyEvent like this way:
```
case KeyEvent.KEYCODE_VOLUME_UP:
                if (action == KeyEvent.ACTION_DOWN) {
                    Bitmap lastImage = Bitmap.createBitmap(fb.getWidth(), fb.getHeight(), Bitmap.Config.ARGB_4444);
                    lastImage.copyPixelsFromBuffer(IntBuffer.wrap(tmpPixels));

                    File sdCard = Environment.getExternalStorageDirectory();
                    File dir = new File (sdCard.getAbsolutePath() + "/my_jpg_test");
                    Log.i("JPCT","My File path" + sdCard.getAbsolutePath() + "/my_jpg_test");
                    dir.mkdirs();

                    String fileName = String.format("The.jpg");
                    File outFile = new File(dir, fileName);
                    FileOutputStream outputStream = null;
                    try {
                        outputStream = new FileOutputStream(outFile);
                        int quality = 100;
                        lastImage.compress(Bitmap.CompressFormat.JPEG, quality, outputStream);
                        outputStream.flush();
                        outputStream.close();

                    } catch (FileNotFoundException e) {
                        e.printStackTrace();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
}
```
And it works!
By the way, obviously, the program becomes extremely slow by using tmpPixels to record the pixel of image on onDrawFrame in this way. Of course I will use some flags to control this process. Thanks again!