Pollinator sharing can lead to heterospecific pollen (HP) receipt, potentially reducing plant fitness. Compatible HP may cause greater reproductive interference than incompatible HP, but the factors determining HP-pistil compatibility and the resulting HP effects are unclear. Moreover, in some apocarpous taxa, the extragynoecial compitum may confer potential HP compatibility, amplifying the risk of reproductive interference by increasing the opportunities for HP tubes (especially closely related HP tubes) to waste more ovules. However, HP-pistil interactions in such plants are understudied. In this study, widely distributed apocarpous Sagittaria trifolia with typical extragynoecial compitum was selected as a representative species, and hand‐pollination experiments were performed with 42 sympatrically distributed and co‐flowering HP donors to systematically access pollen-pistil interaction and HP effects on seed set. Results showed that HP-pistil compatibility was not influenced by species origin (native/alien), pollen size, or aperture number of HP donor. Instead, it was negatively correlated with phylogenetic distance: closely related HP showed higher compatibility, causing greater seed set reduction. Nevertheless, conspecific pollen (CP) sired over 80% of seeds when applied simultaneously with or after HP. Furthermore, in the half-and-half pollination treatments with CP and compatible HP, CP produced significantly more than 50% of seeds. This indicated that pollen advantage and intercarpellary pollen-tube growth of CP enable it to outcompete HP for ovules. Our results provide a good basis for understanding the variation in fitness costs of HP receipt and how apocarpous angiosperms with extragynoecial compitum mitigate interspecific pollen interference.