Abstract: Fission chambers are installed in the measurement channels within the reactor and the signals are transmitted via inorganic and organic cables. They can be used for monitoring the neutron flux of the reactor from startup to full power operation. Aiming at the measurement requirements of the output signal of the pulse ionization chamber, a fully differential pulse preamplifier for a fission chamber is designed. It mainly consists of two charge sensitive loops and a differential circuit stages. The charge sensitive loop uses a low noise field effect transistor as the input stage and a common base circuit as the secondary stage to achieve signal amplification. The differential circuit uses a high common-mode rejection ratio operational amplifier chip to overcome the interference and noise introduced by the front-end cable. The results show that when the fission chamber and preamplifier is connected by 10 m rigid cables and 30 m long flexible cables, the rise time of the output signal is less than 200 ns, the mean square value of noise is 13.3 mV, and the SNR is 30:1. The circuit performance basically meets the requirements of in-reactor neutron flux measurement.