Objective：To investigate the clinical efficacy of losartan potassium in the treatment of cardiac syndrome X（CSX）and to analyze the predictive factors of its therapeutic effect. Methods：A total of 86 female patients with CSX admitted to our hospital from January 2015 to June 2019 were randomly divided into two groups：the treatment group（n=42）received losartan potassium orally for 20 weeks，and the placebo group（n=44）received placebo treatment. The clinical efficacy and predictive factors of treatment effect were compared between the two groups. Results：86 women completed 20 weeks of treatment and repeated the Seattle angina questionnaire（SAQ）assessment. The baseline data of the treatment group and the placebo group matched well. Compared with placebo group，the treatment group was associated with improved frequency of angina pectoris，lower incidence of angina pectoris，better quality of life and body function（P=0.031）. Losartan potassium treatment（P=0.04），Internal carotid artery resistance index（P=0.037）and lower baseline angina freedom（P＜0.001）were important independent predictors of improving angina score. Conclusion：In the absence of obstructive coronary heart disease，losartan potassium therapy can improve microvascular function in CSX women with ischemic symptoms，which is related to the decrease of arterial resistance and angina.