A new method to improve the performance of Brillouin optical time domain reflectometer (BOTDR) system is proposed, by introducing the reported pulsed pre-pump technique into the well known structure based on the consideration of Brillouin spectrum reshaping. In this method, the optical pulse launched into the fiber is provided by electro-optical modulation with a step pulse composed of a pre-pump pulse and a sensing pulse. The reshaping of Brillouin spectrum and the amplification of sensing signal are achieved through the stimulated Brillouin scattering (SBS) interaction between pre-pump pulse and sensing pulse, and the contradiction between spatial resolution and measurement accuracy can be solved, effectively. Meanwhile, the signal to noise ratio and the sensing distance can be enhanced significantly. SBS models under steady-state are solved by transient analysis in the step pulse modulation BOTDR, the optimal design of step pulse is made, and the system performance is compared with those in rectangular pulse and Lorentz pulse modulation structures. The results show that with a step pulse synthesized by a pre-pump pulse with a width of 50 ns and a peak power of 0.2 W and a sensing pulse with the corresponding parameters of 2 ns and 1 W, Brillouin spectrum width can be as narrow as about 44 MHz, and the signal to noise ratio at the end of 10 km single mode fiber can be 7.97 dB higher than that in the rectangular pulse structure. Finally, the feasibility of the proposed method is demonstrated experimentally.