The large-male mating advantage and size-assortative mating are two different size-based patterns, which deviate from random mating in toads. These two pairing patterns may arise due to female choice, male—male competition, male choice, or a combination of these. This study investigated the mating system of Minshan's toad (Bufo minshanicus) from three populations along an altitudinal gradient during two breeding reasons in the northeastern Tibetan plateau. Our study shows that males found in amplexus with females were larger on average than non-amplectant males in two sites with higher operational sex ratios. Similarly, in those sites, males and females found in amplexus maintained an optimal size ratio. These data suggest that male—male competition leads to size-assortative mating in the lack of mate choice (female and male mate choice) by Minshan's toad, as larger males performed higher frequencies for taking-over other low quality ones with amplectant females.