The article presents the results of computer simulation of stress-strain state (SSS) of steel structure of APRS-40 unit tower for underground repair and overhaul of wells. It briefly describes the method of creating tower solid model. The article includes comparative results of stress-strain state changing under various support loads and angles of inclination, as well as in case of incomplete attachment of tower extending section to supports. Creation of solid model and examination of its SSS are performed by KOMPAS-3D computer-aided design system and APM FEM specialized calculation module that implements the finite element method. When modeling SSS, three problems of practical interest for operation of the unit structure are solved: 1) SSS modeling of the tower structure with increase of static loads from 100 to 500 kN; 2) SSS modeling of the tower structure with increase of angle of inclination to the well axis from 0° to 6° and at angles of 9° and 12°; 3) SSS modeling of the tower structure with no attachment of extending section to one or more supports. Following survey results, data was obtained allowing to assess the SSS level at various angles of inclination of the tower to the well axis at various stages of loading during operation. It was found that the structure can operate in the most secure way only without attachment to the rear right support due to the fact that the assurance factor decreases in inverse proportion to operating load increase. The developed model can be used in practice for preliminary assessment of SSS of APRS-40 unit tower structures. The model can be quickly modified and adjusted for various tasks, including modeling of various defects in the tower structure.