Objective To investigate the effect of negative capacity on drainage time,subcutaneous fluid accumulation,etc.after modified radical mastectomy.Methods 80 breast cancer patients after modified radical mas-tectomy.Used computer to produce random number,then 80 patients were randomly divided into the two groups.A group were adopted negative capacity of 200mL and B group adopted 400mL.Both groups used disposable negative pressure drainage balls.Results A group patients′subcutaneous fluid accumulation incidence was 27.91%,skin flap necrosis incidence was 2.33%,indwelling time was (25.00 ±2.59)days.B group patients′subcutaneous fluid accumulation incidence was 8.11%,skin flap necrosis incidence was 0,indwelling time was (12.00 ±2.34)days. Drainage time and subcutaneous fluid accumulation incidence were reduced in B group in comparison with A group, and the differences were statistically significant(P<0.01 or P<0.05).Conclusion High negative capacity is recommended in postoperative drainage of modified radical mastectomy,with a minimum of the equivalence of maximum daily drainage volume.If not,drainage in early postoperative duration will not be sufficient enough to insure complete adherence of skin flap,and postponed extubation and subcutaneous fluid accumulation would be expected.