Objective To explore the application of MR guided hippocampal avoidant whole brain radiotherapy（WBRT）for small cell lung cancer（SCLC）．Methods Thirty SCLC patients with brain metastases who underwent WBRT were enrdled．After routine CT localization was performed,and a head MR was performed in a the same position．T1 weighted images of MR and CT images were accurately fused on the Monaco 5.1 planning system．The entire brain tissue and hippocampus region were delineated． The dose drop areas between the hippocampus and the planned target area were expanded 5mm and 15mm in the three-dimensional direction of the hippocampus,respectively．Three volumetric modulated arc therapy（VMAT）radiotherapy plans were designed for each patient on the Monaco 5.1 planning system based on whether the hippocampal tissue was avoid．The average and maximum doses of hippocampal tissue were observed．Results After the avoidance of hippocampal tissue,the D100 of the three radiotherapy plans reached ≥95%,and there was no significant difference in D100 between the three radiotherapy plans in each case．After setting dose drop areas of 5mm and 15mm for external expansion,the average and maximum doses of the left and right hippocampus were significantly reduced,and there was a significant difference in the comparison between the average and maximum doses in the hippocampus of the three radiotherapy plans．Conclusions MR guided hippocampal avoidant technology and the setting of a 15 mm dose drop area can significantly reduce the dose to the hippocampus in patients with SCLC undergo whole brain radiotherapy．