The thickness of the lubricating film formed between the moving ring and the static ring of the mechanical seal is an important evaluation index for its stable operation.In order to realize the non-contact detection of the lubricating film thickness,according to the characteristics of the mechanical seal lubricating film,an ultrasonic testing model of the contact mechanical seal lubricating film was constructed,and the calculation formula of the lubricating film thickness was deduced.It is pointed out that the mechanical seal of the turbo pump will produce two-phase flow during the operation,and the two-phase problem of the sealing medium must be considered in order to accurately measure the thickness of the lubricating film.A mixed factor of intermediate variables was introduced,and the relationship between mixed factor and the density of the sealing lubrication medium was obtained through experiments,and the mechanical seal lubrication medium density under gas-liquid two-phase lubrication was characterized.The thickness of lubrication film of mechanical seal under two-phase lubrication was measured by ultrasonic method,the calculated values of lubricant film thickness with and without two phases were compared and analyzed.The results show that when the degree of gasification is not high,the calculated value of lubricant film thickness with two phases is closer to the real value,and with the increase of the degree of gasification,the accuracy of ultrasonic detection is reduced.It is necessary to consider comprehensively the change of the parameters such as the acceleration and the acoustic impedance.