This paper presents the quasi-full load method, which has many advantages. Its thinking is simple and direct. Its program is easily carried out, its convergence rate is fast and the optimal result is safe, reliable, economical and practical. This paper gives the mathematical model of the space trusses optimum design in practical project, and points out the shortcomings of optimum method often applied by engineers. The method and the peculiarity of quasi-full load structural design are gone into details.