In order to solve the problem of non?destructive, online, and in?service quantitative monitoring / testing of Brinell hardness of alloy steel component, by building and using an advanced critical refractive longitudinal wave parameter testing device, we have carried out experiments on quantitatively evaluating the near surface Brinell hardness of alloy steel using ultrasonic critical refracted longitudinal waves, measured the critical refracted longitudinal wave propagation characteristic parameters in alloy steel calibration specimens, and calculated the detection parameters required for evaluating Brinell hardness, namely acoustic velocity and attenuation coefficient. In the experiment, we altered the front edges distance between the receiving and transmitting probes and the calibrating specimen microstructure, studied the influence of above changes on the ultrasonic propagation characteristic parameters, confirmed the sensitive detecting parameter for evaluating alloy steel calibration specimen Brinell hardness, and finally established the calibration mapping relationship models between the near surface Brinell hardness of 45^# steel calibration specimens and the critical refractive longitudinal wave characteristic parameters, verified its hardness prediction accuracy. The test result shows that the wave detection parameters are different in sensitivity to the calibrated specimen Brinell hardness, there is a strong correlation relationship between the acoustic velocity and the Brinell hardness, and the error of the establishing Brinell hardness prediction model is less than 10%. The attenuation coefficient cannot be used to evaluate the near surface Brinell hardness of 45^# steel specimens. This study provides some reference for evaluating the Brinell hardness of alloy steel components by ultrasonic non?destructive testing methods.