Populations of Acer species often contain more than three sex phenotypes with complex sexual polymorphism including duodichogamy, protandry and protogyny. We identified the mechanisms that maintain sexual polymorphism in Acer pictum subsp. mono, a temperate tree from northern China, by investigating maternal mating patterns and male reproductive success. We used paternity analyses to estimate rates of outcrossing and disassortative mating, as well as male outcrossed siring success, in a population of A. pictum subsp. mono with uneven sex phenotype ratios (duodichogamous 69.1%, protandrous 19.6%, protogynous 11.3%). We used a pollen-transfer model to investigate whether the unequal ratios of sex phenotypes could be explained by the observed patterns of mating. Most progeny resulted from outcrossing, particularly disassortative among the sex phenotypes. Although the duodichogamous phenotype showed a significant amount of intraphenotypic mating, the frequency did not exceed that of disassortative mating. We detected no significant differences in male outcrossed siring success among the sex phenotypes. The pollen-transfer model demonstrated that sex phenotype ratios could be maintained by the observed mating pattern in the population. Our results indicate that disassortative mating among the sex phenotypes can maintain sexual polymorphism in A. pictum subsp. mono and that ratios biased towards duodichogamy can result from frequent intraphenotypic mating in this phenotype.