Ultra short baseline (USBL) positioning system is widely used in underwater geophysical field survey, acoustic tow fish positioning, and sea oil engineering The precision and accuracy are important technical indexes. Normally, people often care about how to improve the precision of position, but there is no unified method about how to evaluate the precision and accuracy. In most experiments, a beacon as target is moored on the seafloor using buoyancy, and survey positions of the beacon refleatedly in a circle track. The waviness of positioning results is used to evaluate the precision, which is analyzed by a statistical method. This paper analyzes the precision evaluation method based on error ellipse, gives the theoretical formulations, and proves the relationship between observation data and error ellipse. This paper also proposes a precision evaluation method of USBL positioning systems based on long baseline (LBL) triangulation, using the obtained result as the true position to evaluate the accuracy of USBL which can offer suggestions to find system error. Using multiple observations to increase redundancy, the precision is far greater than USBL positioning method. Estimated positions can be used as the true ones to serve as a reflerence in evaluating the accuracy. If the deviation between estimated positions using the USBL and LBL methods is larger than expected, the system needs to be recalibrated. Finally, this paper processes the data from sea experiment. The actual sea trial is processed using the LBL method proposed in this paper. Result shows that the precision of a fixed target is well reflected and the system error is modified further, and thus improves the positioning precision of 0.2%. Result also shows that this method may be of a great application value.