This study investigated the sexual cross-compatibility of the double-flowered Lagerstroemia speciosa ‘Yunchang’, using it as the maternal parent and crossing it with Lagerstroemia speciosa (Deep Purple), Lagerstroemia speciosa (Deep Purplish Pink A), L. indica ‘Red Rocket’, L. indica ‘Pink Velour’, L. balansae, L. siamica, L. tomentosa, and L. indica ‘Red Leaf’ as paternal parents. Through experiments including pollen viability testing, stigma receptivity assessment, hybridization, hybrid seed germination, and fluorescence microscopy observation of pollen tube germination and growth after pollination, the cross-compatibility of double-flowered Lagerstroemia speciosa ‘Yunchang’ during sexual reproduction was systematically analyzed. The results showed: (1) The stigmas of double-flowered Lagerstroemia speciosa ‘Yunchang’ exhibited receptivity throughout the day, peaking between 10: 00 AM and 12: 00 PM when the stigma surface appeared dark blue with abundant bubble formation. (2) The cross between double-flowered Lagerstroemia speciosa ‘Yunchang’ and Lagerstroemia speciosa (Deep Purplish Pink A) demonstrated significantly higher fruit set rate (22.2%), fruit longitudinal diameter (19.45 mm), thousand-seed weight (7.9 g), and seedling emergence rate (8.7%) compared to other combinations. The cross with L. indica 'Red Rocket' produced the highest number of seeds (153 seeds), while no fruit set was observed in the cross with L. balansae. Severe distant hybridization barriers were identified, particularly showing the lowest compatibility between double-flowered Lagerstroemia speciosa ‘Yunchang’ and L. balansae. (3) Fluorescence microscopy observations of combinations with high and low fruit set rates revealed that no pollen germination occurred on stigmas within 0-48 hours post-pollination in low fruit set combinations. In high fruit set combinations, no pollen grain germination was observed at 0 h and 2 h post-pollination; however, at 4 h and 6 h, a few pollen tubes began elongating, accompanied by significant callose deposition on stigmas and pollen tube rupture. Pollen tube elongation became more pronounced at 8 h and 24 h post-pollination. By 48 h, pollen tubes had elongated to the style position, with substantial callose interference and pollen tube distortion. These observations suggest that the sexual hybridization barriers in double-flowered Lagerstroemia speciosa ‘Yunchang’ may originate from extensive callose interference on the stigma.