The invention discloses a low g value capacitive MEMS accelerometer and a modal localization measurement circuit thereof. A capacitive MEMS acceleration sensor comprises a silicon-based mass block, a spring, capacitor plates, X, Y, and Z direction limit blocks, and capacitor plate electrodes. Four differential capacitors are formed by the capacitor plates and mass blocks. Through a first lead wire hole, a second lead wire hole and the capacitor plate electrodes, a differential capacitance signal is introduced into a modal localization circuit. A capacitance measurement system comprises an ASIC modal localization circuit, a reference voltage source, and a multi-phase clock source. The output current and capacitance of the ASIC modal localization circuit are approximately in a linear relationship. When a frequency analysis and data acquisition/retention module carries out sampling to obtain a modal frequency of the modal localization circuit, a response current signal is inputted into a DSP to carry out cache calculation via low pass filtering. A novel differential capacitance MEMS acceleration sensor and a modal localization measurement circuit which detects weak capacitive change are designed, and the high precision weak acceleration change is realized.