Because of there are many factors which influence the length of the culvert, further?more there are interactions between these factors，therefore the calculation of the length of the culvert is rather complex. Based on the spatial analysis,this problem is treated as a spatial analyt?ic problem with two intersecting equations ： (l)a spatial linear equation in which the straight line passes through the inner corner of the cap-beam of the culvert nearby the slope of the. roadbed, and(2) a spatial cruved-surface equation of that slope of the roadbed. By this method,the deduct?ing procedures of the formula for calculating the length of the culvert are very easy, convenient and audio-visual，and thereby the expression formula of the length of the culvert under possible conditions has been obtained. The computer programs are made by using a method of rapid ap?proaching the real value,and the calculating results may be obtained on Computer 486 within only one second.