The deformation of coal during the injection of sorptive gases can be classified into two types: deformation due to the pore pressure and the swelling deformation of the coal matrix induced by adsorption. These types of deformation have not been investigated separately in the previous researches. The main reason is that measuring the radial strain of the coal core directly and accurately is extremely difficult under triaxial stress. This paper demonstrates that by injecting both sorptive and non-sorptive gases into a coal sample with different pore pressures, while measuring the radial deformation of the coal sample with a high precision instrument (± 0.227 mm3), deformations due to the pore pressure and deformations induced by adsorption were separated from each other. The adsorption-induced deformations have an important influence on the effective stress. We studied the relationship between Biot coefficient and the swelling deformation due to the adsorption based on the Biot effective stress theory. The experimental results show that the Biot coefficient is the bilinear function in terms of the volumetric stress and pore pressure, shown as $$\alpha = a + b\Theta + cp + d\Theta p$$ . The data and results obtained in this experiment played a fundamental role in the applications of gas extraction, coal and gas outburst prevention and control, displacement of coalbed methane with CO2, and the establishment of the solid–fluid coupling model.