In view of the 0.8NBT0.2KBT leadfree piezoelectric ceramics used in resonant sensor，the coupling of the thickness stretching vibration and other vibration in thin square piece oscillator which has different length to thickness ratio was researched, the technical parameters of this vibration mode were counted to analyze the practicability of this mode used in resonant sensor. It is determined that the length to thickness ratio of the piezoelectric vibrator with highest coupled and smallest resonant peak is 9.5∶1. Comparing with the same size of QCM, when this resonance body is applied to the sensor, the theoretical sensitivity is reduced 70.1%, while the effective range theory t is improved 129%.