Objective: To understand the relationship between serum 25 hydroxyvitamin D [25(OH)D] levels and social emotional functions in children with autism spectrum disorder (ASD), in order to provide the reference for comprehensive interventions in ASD children. Methods: From January to June 2024, 124 ASD children aged 1-3 who received rehabilitation training at designated rehabilitation institutions in Nantong City, China were selected as the case group, while 124 healthy gender and age matched children who underwent health examinations at the same time were selected as the control group. The study used liquid chromatography-mass spectrometry to measure serum 25(OH)D levels in both groups of children. The Chinese Infant-Toddler Social and Emotional Assessment (CITSEA) was used to evaluate the emotional and socialization functioning of children with ASD, and to explore the relationship between serum 25(OH)D levels and their emotional and social functioning. Results: The serum 25(OH)D levels were lower in the case group [(59.22±19.96)nmol/L] compared to the control group [(85.50±21.59)nmol/L], and the rate of 25(OH)D deficiency or insufficiency (21.77%) was higher than that of the control group (7.26%), with statistically significant differences ( t/χ 2=-7.75, 8.91, P <0.01). The CITSEA evaluation results showed that the scores of the explicit behavior domain, implicit behavior domain, dysregulation domain, and ability domain in children with ASD were (63.37±10.44, 56.29± 9.36 , 57.04±10.65, 38.92±17.91) points, and the abnormal detection rates were 50.81%, 35.48%, 41.13%, and 45.16%, respectively. Among them, the abnormal detection rates of the explicit behavior domain and ability domain were higher in boys ( 57.14 %, 51.02%) compared to girls (34.62%, 23.08%), and the differences were statistically significant ( χ 2=4.18, 6.48, P < 0.05 ). The abnormal detection rates of explicit behavioral domains and dysregulated domains in ASD children with insufficient or deficient serum 25(OH)D (77.78%, 59.26%) were higher than those in the normal serum 25(OH)D group (37.11%, 18.56%), and the differences were statistically significant ( χ 2=14.06, 17.58, P <0.01). Conclusion The serum 25(OH)D levels in children with ASD are significantly lower compared to levels in healthy children, and developmental abnormalities in social emotional functioning are common concurrent problems.