Grain shape, heading date, and amylase content are pivotal traits influencing rice yield, quality, distribution, and regional adaptability. Through our investigation, we identified a mutant, characterized by slender grains, elevated amylose content, and early heading date. Histocytologic scrutiny unveiled heightened cell proliferation in the spikelet hull contributing to the slender grain morphology. The OsSCL26 gene, governing these significant traits, was meticulously cloned via fine-mapping. Phenotypic scrutiny of OsSCL26 knockout and overexpression lines validated its pivotal role in trait regulation. Further analysis disclosed a substitution in the OsSCL26 promoter region, creating a novel binding site for the transcript factor OsbZIP47, thereby modulating its expression in the osscl26 mutant. Functionally, OsSCL26, acting as a serine/arginine-rich SC35-like protein, interacted with U1-70K in vivo and in vitro. OsSCL26 exhibited direct binding to genes implicated in grain shape and carbohydrates, thereby regulating their splicing. Moreover, OsSCL26 showed direct and indirect associations with target RNAs involved in circadian rhythm. Overall, our findings elucidate the mechanism of OsSCL26, an RNA binding protein interacting with splicing factor, as a crucial member of the spliceosome, thereby impacting post-transcriptional splicing and regulating grain shape, heading date, and carbohydrates in rice.