Aiming at the problem of damage to wheat seedlings caused by furrow cutters and failure to evaluate the accuracy of topdressing position in the process of wheat top dressing operation, a precise evaluation system of wheat topdressing based on ultrasonic sensor was studied to evaluate the accuracy of topdressing machine in real time. The system was composed of a work trajectory detection part, a data collection part, and a data analysis and evaluation part. The ultrasonic sensors were installed on topdressing machines to scan the wheat line and detect the operation track of the topdressing machines. The collected data were sent to the controller from each ultrasonic sensor by CAN bus in real time to keep tracking topdressing machine operation. The abnormal data were filtered out and eliminated threshold filter algorithm. The optimally current voltage was estimated by the Kalman filter algorithm. The offset distance of the fertilizing machine and the accuracy of the topdressing machine were calculated according to the difference between the target voltage and the optimal estimated voltage. Field experiment showed that the offset misjudgment rate was 9%, the maximum error distance was 3.15cm, the error distance during 2cm accounted for more than 90%, and the average standard deviation was 2.10cm, which was consistent with the actual operating conditions. In conclusion, the evaluation system provided a new means of measurement for evaluating the accuracy of wheat topdressing.