Objective To explore the role of ASA score (American Society of Anesthesiologists physical status classification) and operation time in risk assessment of surgical site infection (SSI) in patients with colorectal cancer. Methods Clinical data of patients undergoing colorectal cancer surgery from January 2018 to December 2019 were retrospectively collected, correlation between ASA score and post-operative SSI was analyzed, receiver opera-ting characteristic (ROC) curve was used to test the efficiency of ASA score in risk assessment of patients with SSI, and to compare the effect of length of operation time on SSI under different ASA grades. Results A total of 430 patients undergoing surgery of colorectal cancer were included in study, 243 cases of colon cancer and 187 cases of rectal cancer, 238 males and 192 females. 57 patients had SSI, incidence of SSI was 13.26%. With the increase of ASA score, incidence of SSI in patients gradually increased. ROC curve analysis showed that area under the curve of ASA score was 0.706, which was statistically significant (P < 0.05). For the medium-risk ASA score group, incidence of SSI in patients with operation time ≥ 180 minutes was higher than those with operation time < 180 minutes, difference was statistically significant (P < 0.05). Conclusion ASA score can assess the risk of post-operative SSI in patients with colorectal cancer, surgeons should pay attention to and improve the systemic status of patients before surgery, and shorten the operation time as much as possible to reduce the incidence of SSI.