Could you please give me an advice

Thanks Paleo12 and Sfar, very much appreciate your help.

The computer and mobile phone connected to the receiver is not a problem now, but I suddenly found that the subwoofer sound is very small, when I use the mobile phone to use the 3.5mm to RCA cable to connect to the receiver to play music or listen to the FM radio, The left and right speakers can hear the bass, but the subwoofer is similar to none, the sound is very weak, I don't know why. Is the subwoofer broken?

You can go headphone out to aux in. Seriously crappy. Or USB out to a DAC to aux in. Garden variety crappy. But I believe that you will be impressed.

Cannot advise on sub without info.

On many receivers, if you use a left/right analog source, it will NOT process the sound and send the bass to the subwoofer.  It will just pass-through the left/right signal to only the main left/right speakers. You may want to start reading the documentation for your Onkyo receiver.  You might be able to switch to a different decoding mode (such as Dolby Music or DTS:Neural:X).

Thank you very much for the help and reply above, I will study the manual again. thanks