Sm-Co based alloys are best candidate materials in high-temperature applications, such as national defense industry, aerospace engineering, and microwave communications, etc. The addition of appropriate alloying elements may help to improve the performance of Sm-Co alloys, thereby meeting the acquirements in the above-mentioned fields. The first-principles calculation method is suitable for screening candidate doping elements and it can provide a theoretical basis for the design of Sm-Co alloys. In this work, a first-principles calculation model of SmCo5 alloy with main group doping elements is established. Taking Al, Ga, In, and Sn as examples, the influence of intrinsic characteristics of doping elements, doping concentration and temperature on the phase stability and magnetic properties of SmCo5 alloy are studied. The bonding interactions between the doping elements and Co atoms are revealed based on the electronic structures. Combined with the analysis of electronic structures, such as bond population, charge density, differential charge density and density of states, the microscopic mechanism of the effect of different elements on the stability of SmCo5 was clarified. The calculation indicates that the physical and chemical properties of doped main group elements and the size of the vacant space in the doping systems are two main factors that influence the occupation site of the doping elements. Al and Ga are beneficial to the stability of SmCo5 system. Moreover, the occupation sites probability of Al varies insignificantly with temperature, which indicates that the SmCo5 system doped with Al may be applied in applications with a wide temperature range. Regarding to the magnetic properties, the SmCo5 system doped with In has a relatively high magnetic moment, although the total magnetic moment of the SmCo5 system always decreases with the addition of the studied main group elements. The main reason is that In has a relatively large atomic radius, which may induce lattice distortion and result in the increase of magnetic moment between Co atoms. Therefore, the decrease of the total magnetic moment of the doping system can be compensated. Based on the above calculations, Al and In are selected as the candidate elements which are beneficial to the stability and magnetic properties of SmCo5 based alloys. Moreover, the optimal doping concentration ranges of Al and In are predicted.