Variation in size may influence the abundance of visitors and reproductive allocation for cushion plants in extreme alpine environments. To assess the effects of plant size on the abundance of main visitors and reproductive allocation in Thylacospermum caespitosum populations at two altitudes, the abundance of the visitors, visiting frequency, the total number of flowers, number of fruits, number of unseeded flowers, and reproductive allocation were investigated during the period of reproductive growth. Concurrently, the effects of plant size on the visitors’ contributions to fruit setting rate were assessed by a bagging experiment. Our results showed that flies (Musca domestica and Dasyphora asiatica) were the main pollinating insects of T. caespitosum, and they could obviously facilitate (p < 0.05) the fruit setting rate of this cushion plant. Seed set and floral visitation were significantly influenced (p < 0.001) by plant size. Moreover, the reproductive allocation and fruit setting rate of T. caespitosum was influenced (p < 0.001) by plant size. More biomass was allocated to reproduction in plants of greater diameter. There is an increase in reproductive success (increases of fruit number with the increase in plant size) in relation to plant size. In conclusion, the extent of M. Domestica and D. asiatica to facilitate the fruit setting rate mainly de‐ pended on the size of T. caespitosum. Size‐dependent reproductive allocation occurred in T. caespitosum and was the chief factor affecting the contribution of flies to fruit setting rate. These traits reflect the reproductive fitness of T. caespitosum related to plant size in extreme alpine environments.