i think this was not caused by your PHD degree, and this may be the result from the eduction system of china.
you have a lower salary work, then you go to the university and finish your phd, and you think you can get more after getting the phd degree, finally, you find you make a big mistake in your life, nowadays, i think this is normal in china.
Everything will be changed, work hard, fight, fight and fight . i wish you will have a good future .