The master equation of the density operator in the system of atom interacting with thermal reservoir was concisely solved by virtue of Ket-Bra entangled state. Analytical solution expression of the master equation was given. Considering that each of atoms interacts with thermal reservoir respectively, negativity was adopted to quantify the degree of tripartite entanglement among three atoms, and properties of tripartite entanglement among atoms were investigated. The influences of mean photon of the thermal reservoir and atomic spontaneous emission rate on properties of tripartite entanglement were discussed by using the numerical method. The results show that the tripartite entanglement among atoms decays accelerating with the increase of mean photon of the thermal reservoir and atomic spontaneous emission rate.