Objective To investigate the appropriate time of intratympanic injection of methylprednisolone as combined or remedial administration for the treatment of sudden deafness of flat descending type. Methods Clinical data of patients with unilateral sudden deafness of flat descending type hospitalized in our department from July 2019 to Oct 2021 were collected. According to the inclusion and exclusion criteria, 145 cases were finally enrolled, including 62 cases in the control group treated with systemic medication and 83 in the treatment group receiving intratympanic injection of 20 mg methylprednisolone (once every other day, for 5 times) on the basis of systemic medication. The time interval between the onset of disease and the first intratympanic injection was recorded, and the treatment group was subdivided into “≤10 days group” and “> 10 days group” according to the cut-off point of 10 days. The mean hearing thresholds of all patients at the initial stage of onset (≤3 days), 1 month and 6 months were obtained. And then the hearing improvement in each group at different times was recorded and statistically analyzed. Results At the beginning of treatment, there were no significant differences in the average hearing threshold among all groups(P>0.05). At 1 month, the mean value of hearing improvement in the “≤10 days group” was higher than that in the control group, the difference was statistically significant (P＜0.05). However, the differences in the mean value of hearing improvement between “>10 days group” and the control group, or between “≤10 days group” and “> 10 days group” were statistically insignificant (P>0.05). At 6 months, the mean value of hearing improvement in the “≤10 days group” was significantly higher than that in the “>10 days group”(P=0.002). Conclusions Compared with systemic therapy alone, the combined intratympanic glucocorticoid injection is conducive to hearing improvement for patients with sudden deafness of flat declining type. Early initiation (within 10 days of onset) is recommended to achieve preferable long-term efficacy.