Abstract: Objective To explore the conversion relationship between subjective and objective pain assessment tool scores in critical ill adults. Methods The convenience sampling method was used to select 115 critically ill adults who were admitted to ICU, the Affiliated Hospital of Xuzhou Medical University from March 2021 to October 2021. The Numerical Rating Scale (NRS), Critical-Care Pain Observation Tool (CPOT) and Behavioral Pain Scale-China (BPS-C) were used to assess patient pain at resting (n=70) and during painful procedures (n=86). The correlation and conversion relationship between NRS and CPOT as well as NRS and BPS-C were analyzed. Results The CPOT scores were significantly correlated with NRS scores at resting and during painful procedures (P<0.05). Then, the following linear regression equations between CPOT and NRS were established: CPOT=0.122+0.442×NRS (R²=0.452) at resting and CPOT=0.932+0.486×NRS (R²=0.554) during painful procedures. Furthermore, BPS-C scores were also significantly correlated with NRS scores at resting and during painful procedures (P<0.05). Then, the following linear regression equations between BPS-C and NRS were established: BPS-C=2.774＋0.415×NRS (R²=0.420) at resting and BPS-C=3.288+0.519×NRS (R²=0.501) during painful procedures. Conclusions The linear regression equation can be used to convert the scores between NRS and CPOT as well as NRS and BPS-C, so that the pain assessment scores of critical ill adults can be standardized under the same scale. The regression equation established by NRS and CPOT has a better fitness, and CPOT is more suitable for pain assessment in critically ill adults.