When veterinary B-ultrasound was used to detect pregnancy in dairy cows, the accuracy rate of diagnosing empty sac (55.56%) was the lowest in 20-22 d dairy cows, and the suspicious diagnosis rate (28.57%) was the highest. The reason is that the diameter of the embryonic sac of early pregnant dairy cows is generally less than 1 cm, and the resolution of medium-performance B-ultrasound is not enough. The experimenter could not get an accurate and clear image due to interference from various foreign objects during the examination, and thus could not make an accurate judgment. The accuracy rate of diagnosing pregnancy in the three time periods of 20-28 d was significantly higher than the accuracy rate of diagnosing empty sac, indicating that veterinary B-ultrasound diagnosis is better than that of diagnosing empty sac in the early stage of dairy cow mating, because veterinary B-ultrasound examination can confirm the diagnosis as long as a clear embryonic sac image is seen, which is generally accurate. However, due to the differences in individual dairy cows and B-ultrasound performance, it is not possible to accurately determine empty sac if a clear embryonic sac image is not seen. At 29-31 days, the diagnostic accuracy was 100%. During this period, the diameter of the embryonic sac was about 2 cm, and most of the embryos could be seen in the blastocyst, and the observation effect was good. Therefore, 29-31 days was selected as the most ideal time for pregnancy diagnosis. At 20-28 days, the diagnostic accuracy of young cattle (81.25%) was significantly higher than that of multiparous cattle (68.09%), especially the diagnostic accuracy of young cattle was as high as 100%, indicating that the early pregnancy diagnosis effect of veterinary B-ultrasound in young cattle was better than that in multiparous cattle. The reason may be that multiparous cattle generally have a deeper uterus, slightly sagging, thicker uterine wall, and more fat accumulated nearby, while the uterus of young cattle is shallow, without sagging, and there is no fat accumulation near the uterus. The ultrasound image of young cattle is clearer than that of multiparous cattle during veterinary B-ultrasound diagnosis, and the confidence of diagnosis is higher. Therefore, the time of pregnancy diagnosis after mating of young cattle can be appropriately advanced. The diagnostic accuracy of both young cattle and multiparous cows aged 29-40 days was 100%, indicating that both achieved ideal results in Veterinary Ultrasound diagnosis during this period.