To implement a function of detecting phase change at a high precision in MIT system, a phase-change-detecting unit based on RF Lock-in Amplifier SR844 was designed at the frequency of 1 MHz. The unit provided a gain range of 3.52 dB – 61.1 dB and a phase resolution by 0.02 deg. And an experiment with neuron cell model from SD rat cortex was made to detect phase change produced by edema neuron cell model and normal neuron cell model respectively. The results of the experiment showed that the phase change produced by edema neuron cell model was smaller than that by normal neuron cell model and that the unit had good capability of detecting the phase change produced by edema neuron cell model and normal neuron cell model effectively. These may be of great help for further cerebral edema-detecting work.