A silicon nanobelt field effect transistor (SiNBFET) device was proposed as an alternating platform of ultrasensitive biosensor, and apply to the label-free detection and early diagnosis of the prostate specific antigen (PSA). The designed SiNB-FET molecule sensor demonstrated real-time, label-free, and high-selective properties in detecting biomolecules. The novel back-gate SiNB-FET was fabricated by using the state-of-the-art complementary metal oxide semiconductor (CMOS) manufacturing technology. The shrank nanobelt structure with high surface-to-volume ratio and individual back-gate controlling was achieved by the local-oxidation of silicon (LOCOS) process. The probe molecule was sequentially immobilized onto the device surface for the purpose of target molecule sensing. Those molecules bearing with charge characteristics significantly influenced the charge carrier in the device channel. Hence, the target PSA can be easily detected from the shift of device's electrical property. In this research, the operating condition of device's gate controlling voltage was carefully studied. In addition, the molecular amplification method was developed to enhance the method's sensitivity. Finally, real samples from the hospital site were evaluated to characterize the concentration. We have demonstrated the detection capability of PSA by the SiNB-FET, and results show that the nanobelt biochip will be applied to the clinical diagnosis, and verify its feasibility on the ultrasensitive diagnosis of prostate cancer in the future.