Abstract: Composite fire protection is the combination of fire protection board and fire protection cotton covered on the surface of steel members. Compared with the coating fire protection, composite fire protection has some advantages, such as dry construction, long durability and decoration integration, and the like. Taking square steel tubular columns as the research object, a new configuration of composite fire protection was proposed and two full-scale fire resistance tests were performed under a load ratio of 0.6. The specimen was protected by the combination of fire protection gypsum board and aluminum silicate cotton. The local buckling failure of the whole cross section was identified and the fire resistance limit was 147 min. A simplified theoretical model was proposed to effectively solve the temperature rise of the square steel tubular column with composite fire protection exposed to 4-side fire. In this way, the two-dimensional heat transfer problem was simplified to an equivalent one-dimensional heat transfer one, and the control equation of the theoretical model was derived. Moreover, based on the traditional finite element simulation, the number of typical calculation examples of square steel tubular columns was increased from 2 to 23. After comparing the results of these calculation examples, it was found that the simplified theoretical model showed a good accuracy, with an average error of 6.03% for the temperature rise of steel column in fire. Meanwhile, the calculation efficiency was improved by approximately 35.7 times on average. A simplified design method for the fire resistance of square steel tubular columns with composite fire protection was provided on the basis of the critical temperature method obtained from the Chinese “Code for fire safety of steel building structures”. The high accuracy of the simplified fire resistance design method was verified by the comparison results of the calculation examples, with the average error of the fire resistance limit of 4.46%. Hence, the fire resistance design of such steel members was implemented.