In order to capture more energy, a variable-sectional cantilever piezoelectric energy harvester with groove was proposed in this paper. Firstly, the beam form is changed according to a given exponential function, and the groove is dug inside the beam. Secondly, the corresponding vibration equation is deduced, and the expression of vibration mode is determined by referring to relevant literature, and then the corresponding characteristic equation is established, the expression of voltage and output power is derived, and the optimal exponent is determined according to the voltage and output power. The results show that under the premise of keeping the beam length unchanged, the larger the bending degree coefficient of the tapered beam, the larger the output voltage and power, but the larger the characteristic frequency of the system.