Abstract: Tempering and hydrogen charging were carried on a 2.25Cr-1Mo steel used for hydrogenation reaction, and then to study the effect of hydrogen embrittlement and temper embrittlement on embrittlement properties of the steel by impact testing, Auger electron spectrometer testing and scanning electron microscopy testing.Basing on grain boundary segregation theory, the amount of P atom segeration at grain boundaries was calculated after different tempering and hydrogen charging treatments. The results show that the microstructure and hardness could not be used to assess the embrittlement degree of the tested steel. With the increase of tempering time during tempering temperature range, the temper embrittlement gradually increased, and hydrogen charging enlarged embrittlement degree. The effect of combined effects of hydrogen embrittlement and temper embrittlement on embrittlement degree was much more than that of temper embrittlement alone. Hydrogen charging amplified diffusion coefficient of P atom, and thus aggravated element P segeration at grain boundaries, and this was the main reason of hydrogen charging aggravating temper embrittlement.