[Objective]To investigate the infection and vertical transmission of Mycoplasma wenyonii in dairy cows,and to provide reference for the prevention and control of Mycoplasma wenyonii in dairy cows.[Method]From March 2021 to July 2022,blood samples of 2 562 dairy cows from 9 dairy farms in Xianghe County were collected,among which 781 dairy cows were diagnosed with pathogens by direct blood smear,droplet pressing method,Giemsa stain,Wright's stain and PCR.Mycoplasma wenyonii was detected in 810 cows at different ages and 971 cows in different seasons by droplet pressing method and PCR,respectively.30 pregnant cows and newborn calves with positive pathogen were selected to detect vertical transmission infection.[Result]The positive rates of Mycoplasma wenyonii infection in 781 dairy cows were 75.54%,49.81%,55.57%,54.03% and 46.61% by direct blood smear,droplet pressing method,Giemsa stain,Wright's stain and PCR,respectively.The positive rate of direct blood smear was significantly different from that of other methods.The positive rate of blood suspension by droplet pressing method was the closest to that of PCR.The positive rate and incidence of Mycoplasma wenyonii infection in lactating cattle were significantly higher than those in calves and young cattle.The positive rate and incidence of PCR detection of Mycoplasma wenyonii infection in cows at different age stages were as follows：lactating cattle>calves>young cattle.The positive rate of Mycoplasma wenyonii was the highest in summer cows,and the negative rate of PCR detection was 60.85%,which was significantly different from other seasons.The incidence was the highest in summer（10.21%）,but there was no significant difference between seasons.[Conclusion]The clinical diagnosis can be made by droplet pressing method and PCR.The infection rate is high but the incidence is low,which is closely related to age and season,and the pathogen can be transmitted to calves through vertical transmission.