As the sensitivity of a piezoelectric mass sensor directly depends on the structural frequency variation induced by the mass added. This paper proposes a structure designing method for improving the measuring sensitivity of the mass sensor by using a structure which consists of a symmetrical trough (I-shaped cross-section) cantilever and piezoelectric films. Then, we design and fabricate a novel piezoelectric resonance micro-mass sensor. Considering the influences of the section shape, natural frequency and the vibration mode on the sensitivity, an analytical model is established for analyzing the frequency variation caused by the micro particles. With the same geometric parameters, the simulation and experiments are performed for the sensors with the I-shaped cross-section cantilever and the rectangular section cantilever. It shows that the first order natural frequency of the I-shape cross-section cantilever is 1 851 Hz, and that of the rectangular section cantilever is 1 610 Hz. Moreover, corresponding sensor sensitivities are 3.12×104 and 1.5×104 Hz/g, respectively, and the former is twice of the latter. The method is feasible and effective for improving the measuring precision of mass sensors.