We designed an ultrasonic testing instrument that consisted of a single-chip microcomputer module, a digital display module, and an ultrasonic sensor module, which conveniently eliminated the troubles faced by the traditional Jolly’s scale. For comparison purpose, three linear springs’ stiffness factors were measured by Jolly’s scale and by our ultrasonic testing instrument. We found that our instrument could more conveniently and in real time display the distance values between the ultrasonic ranging module and the horizontal bottom plate when loading different weights. By processing these distance data, we found that our instrument was more convenient for obtaining the linear springs’ stiffness factors and that the results were more accurate than those of Jolly’s scale. This study verified that our instrument can accurately realize the performance of Jolly’s scale under diverse temperatures and humidity levels with high data reliability and perfect stability.