A method for improving hardness of an inner gear comprises the following steps that firstly, the integral inner gear is subjected to solid solution treatment, the solid solution treatment temperature is 400-450 DEG C, and the heat preservation time is 4 h; secondly, after solid solution treatment is finished, quenching treatment is conducted on the outer circle face, the upper end face and the lower end face of the inner gear, and the quenching temperature is 50-60 DEG C; thirdly, after the second step is finished, the gear face of the inner gear is subjected to quenching treatment, and the quenching temperature is 70-70 DEG C; fourthly, after the third step is finished, the inner gear is subjected to aging treatment, the aging temperature is 250-280 DEG C, and the heat preservation time is 3 h; fifthly, natural cooling is conducted for 2 h; and sixthly, an induction heater is utilized for conducting heating treatment on the outer circle face, the upper end face and the lower end face of the inner gear, the heating temperature is 200-220 DEG C, and the heating time is 1 h; and sixthly, natural cooling is conducted to the room temperature. By means of the method, the hardness of the inner gear can be effectively improved, the inner gear can better resist impacts, and the service life of the inner gear can be better prolonged.