To explore the interactions between cervical length (CL) and placenta accreta spectrum (PAS) on severe postpartum hemorrhage (SPPH) in patients with placenta previa.A retrospective case-control study was conducted at four medical centers in China, and 588 patients with placenta previa were included. The logistic regression analysis and restricted cubic splines (RCS) were used to evaluate the association between CL and SPPH. Furthermore, the joint effect of CL and PAS on SPPH was assessed, and the additive and multiplicative interactions were calculated.After adjusting for potential confounders, the negative linear dose-response relationship was confirmed by RCS, and the change of odds ratio (OR) was more significant when CL ≤2.5cm. The risk of SPPH was significantly higher when CL≤2.5cm co-existed with placenta increta/percreta than CL≤2.5cm or placenta increta/percreta existed alone [aORCL≤2.5cm&placenta accreta/non-PAS =3.40(95%CI, 1.37-8.45); aORplacenta increta/percreta&CL>2.5cm =4.75(95%CI, 3.03-7.47); aORCL≤2.5cm&placenta increta/percreta =14.51(95%CI, 6.08-34.64)], and there might be additive interaction between CL and placenta increta/percreta on SPPH (attributable proportion due to interaction=50.7%, 95%CI, 6.1%-95.3%).If CL was routinely performed during PAS evaluation, the increased OR of short CL and PAS can allow better patient preparation through counseling.This article is protected by copyright. All rights reserved.