Abstract: The mechanics of layered elastic systems is the theoretical basis for the design and calculation of multi-layer flexible pavement. The BISAR program is a world-renowned mechanical calculation program for multilayered elastic systems. For the first time at home and abroad, an obvious error in the BISAR source program has been demonstrated through the derivation of mechanical calculation formulas and numerical calculations. Due to the existence of this error, when using the BISAR program to calculate a layered elastic system under the action of unidirectional horizontal load uniformly distributed on a circular area, the calculation results of \tau _\theta z1 and \tau _zr1 of points located within the range of the load circle on the surface of the layered elastic system are both incorrect. In the 50 year history of the BISAR program, this is an unprecedented discovery .The above research work provides valuable information for further modification and improvement of BISAR source programs.