For the disadvantage in modeling at complex areas, advancing-frontal triangulation method was firstly applied in building numerical model of FLAC3D software. Suitable date structure and algorithm based on Fish program language was mainly studied. The position criterion of adjacent edges to current edge is brought in to improve the efficiency of algorithm; equilateral triangle or isosceles triangle mesh is generated to improve elements’ quality; element size is controlled by simple mathematics means so smooth graded triangular mesh is constructed. Through the research, the first independent FLAC3D pre-process is developed successfully, without relying on other modeling software. Application example shows the feasibility of the method.