Objective To analyze the obvious and hidden blood loss after operations in the posterior lumbar intervertebral fusion(PLIF) by two drainage methods, and compare the effect of the two kinds of drainage.Methods From October 2008 to December 2012, retrospective analysis of 108 patients undergoing treatment of the PLIF was carried out. According to the kind of drainage method after PLIF, they were randomly divided into two groups. 56 patients received ordinary drainage method, 52 patients underwent low suction drainage method. Statistics was applied to comparison analysis of the two groups in terms of total blood loss,visible blood loss,hidden blood loss,and transfusion.Results In the ordinary drainage group: the mean blood loss of (860.62?375.45) ml was found in 56 cases after operation; the mean total blood loss was(1415.10?592.66) ml while the hidden blood loss was (516.73?314.71) ml,36% of the total loss. In the low-suction drainage group: the mean blood loss of (794.17?337.19) ml was found in 52 cases after operation; the mean total blood loss was(1251.53?540.99)ml and the hidden blood loss was (457.36?332.79) ml,35% of the total loss. There were no statistical differences between the two groups.Conclusions The ordinary drainage and the low-suction drainage may have no difference in hidden blood loss after operations.