Abstract: Based on control and error analysis theory, the accuracy of injection weight in the injection-molding process was studied by an online measurement method, and their influence factors were investigated for the samples with different sizes using statistical error algorithm, followed by a comparative analysis. To ensure a real-time efficiency and high accuracy for this test method, the sampling capacity was taken as 5, 10 and 15 for three sets of experiments, and the sampling cycle points were updated successively and recursively. The test and calculation methods were further verified by a modified experimental equipment. The optimal sample size was determined to be 10, and the optimum algorithm was confirmed as the standard deviation one. The research results indicated that the measurement method developed by this work could provide an effective evaluation for the quality accuracy of injection products on the basis of online measurement of ultrasonic melt density, and the test accuracy was in the range of 0.50 %~1.50 %, verifying the feasibility of this measurement method.