Abstract: Aiming at the instability problem of porous functionally graded material （PFGM） sandwich beams under uniform thermal loading， based on the Timoshenko first-order shear theory， von Karman geometric nonlinearity and physical neutral plane concepts， the nonlinear governing equations for PFGM sandwich beams under uniform thermal loads were derived. The analytical solution of the nonlinear integral-differential boundary value problem was obtained by using the Nayfeh semi-inverse solution method， and the type and mechanism of instability of PFGM sandwich beams under uniform thermal loading were investigated in conjunction with the minimum free energy principle. The results showed that， under uniform thermal loading， the C-C PFGM sandwich beams exhibited bifurcation instability； while for the S-S PFGM sandwich beams， the influence of additional moment was induced by the variation of the gradient index， the instability type transformed from bifurcation instability to snap-through.