Background: Active participation in high-fidelity simulation remains stressful for residents. Increased stress levels elicited during such simulation impacts performance. We tested whether relaxing breathing, paired or not with cardiac biofeedback, could lead to enhanced performance of residents during simulation. Methods: This randomized pilot study involved the fifth-year anesthesiology and critical care residents who participated in high-fidelity at Lyon medical simulation center in 2019. Residents were randomized into three parallel interventions: relaxing breathing, relaxing breathing paired with cardiac biofeedback, and control. Each intervention was applied for five minutes immediately after the scenario briefing. The primary endpoint was the overall performance during the simulation rated by two blinded independent investigators. The secondary endpoints included component scores of overall performance and changes in psychological states. Results: Thirty-four residents were included. Compared to the control group, residents in the relaxing breathing (+ 7%, 98.3% CI: 0.3 to 13.7, P = 0.013) and relaxing breathing paired with cardiac biofeedback (+ 8%, 98.3% CI: 0.82 to 14.81, P = 0.009) groups had a higher overall performance score. Following the interventions, compared to the control group, stress level was lower when participants had performed relaxing breathing alone (P = 0.029) or paired with biofeedback (P = 0.035). The internal relaxation level was higher in both the relaxing breathing alone (P = 0.016) and paired with biofeedback groups (P = 0.035). Conclusions: Performing five minutes of relaxing breathing before the scenario resulted in better overall simulation performance. These preliminary findings suggest that short breathing interventions are effective in improving performance during simulation. Trial registration: The study protocol was retrospectively registered on clinicaltrials.gov (NCT04 141124, 28/10/2019).