Objective: To investigate the level of vitamin B12 in children with autism spectrum disorder(ASD), and provide a theoretical basis for early detection and drug treatment of ASD. Methods: A total of 89 ASD cases and 89 matched controls were collected. The levels of urinary methylmalonic acid (MMA) and serum vitamin B12, Transcobalamin Ⅱ (TCN2) were determined by enzyme-linked immunosorbent assay (ELISA). TCN2 gene rs1801198 was genotyped by SNaPshot. Results: The serum levels of vitamin B12 and TCN2 in children with ASD [(369.08±131.88)pmol/L, (1.56±0.16)ng/mL] were significantly lower than those in the control group[(485.16±200.33)pmol/L, (1.71±0.17)ng/mL](t=-5.47, -5.92, P<0.05). The level of MMA in urine of ASD children [(758.97±106.96) ng/mL] was significantly higher than that in the control group[(693.66±121.72)ng/mL](t=3.94, P<0.05); The genetic polymorphism of rs1801198 locus was not associated with the risk of ASD(P>0.05), and there was no significant correlation with serum TCN2 level(F=1.16, P>0.05). Conclusion ASD children are at a potential deficiency of vitamin B12 and should strengthen their nutritional interventions while conducting ASD interventions.