According to the requirement for the concentration gradient generator (CGG) in the chip laboratory, a method of moving focal plane front and back exposure to fabricate SU-8 photoresist microstructure is developed for fabricating CGG with vertical sidewalls. In the method, the thickness of SU-8 photoresist is divided into multiple layers according to the focal depth, and the focal plane moves one layer down with each exposure. When the number of exposed layers reaches half of the total layers, the sample is turned over, and the same exposure way is repeated. At last, the photochemical reaction channel inside SU-8 is formed, and SU-8 is fully exposed. Finally, polydimethylsiloxane (PDMS) CGG is fabricated using the SU-8 microstructure. The results show that the sidewall of the SU-8 microstructure is vertical, without "T" shaped structure, and the channel height of the SU-8 microstructure is 49.4 μm. The sidewall of PDMS CGG is also vertical with the channel depth of 49.3 μm, fully meeting the vertical requirements of CGG sidewalls.