The pH value has been one of the keen parameters in the cell culture. And the optical measurement method especially the optical method using optical fiber has been widely used due to its small volume and high accuracy. In this work we design a phase detection system to detect the phase difference between the signal and reference signal in the Dual Lifetime Referencing Method. The modulation frequency of the DLR method ranges from 10 kHz to 60 kHz and the system we designed can detect phase difference from the frequency of 1 kHz to 100 kHz. The error can be as small as 0.2%.