The principle of geometric analysis was adopted in this paper to calculate the ultrasonic testing accuracy of log samples containing different sizes of internal decay. The relationship between the number of ultrasonic emission points and the detection accuracy of log internal decay was analyzed. And the number of emission points that can effectively test the internal decay defect was determined. Results show that when the defect area of the log is fixed, the testing accuracy can be improved as the ultrasonic emission points increase. When the diameter of the log is 30cm, and the ratio of deficiency in the cross-section of the log is between 2.7% to 44%, at least 4 emission points should be selected for the testing accuracy to reach 90%.