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Readout electronics is developed for a prototype time-of-flight (TOF) ion composition spectrometer for in situ measurement of the mass/charge distributions of major ion species from 200 to 100 keV/1e in space plasma.By utilizing a constant fraction discriminator (CFD) and time-to-digital converter (TDC),challenging dynamic range measurements were performed with high time resolution and event rates.CFD was employed to discriminate the TOF signals from the micro-channel plate and channel electron multipliers.TDC based on the combination of counter and OR-gate delay chain was designed in a high-reliability flash field programmable gate array.Owing to the non-uniformity of the delay chain,a correction algorithm based on integral nonlinearity compensation was implemented to reduce the time uncertainty.The test results showed that the electronics achieved a low timing error of ＜ 200 ps in the input range from 35 to 500 mV for the CFD,and a time resolution of ～ 550 ps with time uncertainty ＜ 180 ps after correction and a time range of 6.4 μs for the TDC.The TOF spectrum from an electron beam experiment of the impacting N2 gas further indicated the good performance of this readout electronic.