Objective To study the value of high frequency ultrasound in knee arthroscopic synovitis and synoviectomy. Methods From January 2017 to February 2018, 30 patients with rheumatoid arthritis were studied. All patients were examined by ultrasonography and MRI. The results of MRI and ultrasonography were analyzed, and the relationship between synovial thickness and the condition and prognosis was also discussed. Results There was no statistical difference between the preoperative high-frequency ultrasound and MRI on the stage of cartilage lesions in RA patients (P＞0.05). Before and after treatment for 6 months, 14 months, ultrasound and MRI measurement for synovial thickness was not statistically significant (P＞0.05). In 1 months and 3 months after treatment, the ultrasonic testing results below the detection results of MRI (P＜0.05). The levels of ESR, CRP, CCP, VAS and RF were lower than those before treatment (P＜0.05). The knee function score of Lysholm after treatment was significantly better than those before treatment (P＜0.05). The ultrasound measurement of synovial membrane thickness was positive related to ESR, the positive rate of RF and CCP positive (P＜0.05). The ultrasound measurement of synovial membrane thickness was negative related to Lysholm knee function scores (P＜0.05). The ultrasound measurement of synovial membrane thickness was not related to CRP and VAS score (P＞0.05). At 6 months after treatment, patients with lower synovial thickness had lower ESR, CCP, and VAS scores, and Lysholm knee function scores were higher (P＜0.05). Conclusion High frequency color Doppler ultrasound has good diagnostic value of knee joint lesions of rheumatoid arthritis at the same time. It is important to evaluate the prognosis.