The construction of power grid infrastructure in mountainous areas has the problems of difficult transportation and high cost, especially heavy equipment cannot climb the narrow and steep mountain road. In this paper, variable density method (SIMP) is used for lightweight design of crawler drill chassis to meet the needs of transportation and drilling in mountain foundation construction. Three-dimensional software is used to establish the model of the frame longitudinal beam and its connection, and its operating conditions are analyzed to obtain its stress under different working conditions. Then the finite element analysis software is used to analyze its deformation and stress distribution. The thin-wall optimization of key longitudinal beam components is carried out by using variable density method. The objective function is set to minimize material volume, and the constraints are defined as maximum deformation and stress constraints. The optimization results show that the optimized chassis stringer mass is reduced by 25.67%, the structural stiffness is enhanced, and the first-order modal frequency is significantly increased. The performance of the optimized structure meets the design requirements under actual working conditions, and provides an effective technical path for the lightweight design of the chassis of the crawler drill.