A novel cavity for improving the beam quality of diode direct side-pumped Nd∶YAG laser was proposed. The enfolded cavity consisted of three parts, a Porro prism, a high-reflectance mirror and a output coupler mirror. It made full use of the symmetry proportion of uneven gain profile to provide better matching between the gain volume and mode volume, so that the better quality beam and the higher optical-optical slope efficiency were obtained. The enfolded cavity was proved to be high effective by the results of experimentation and simulated calculation. For the quasi-CW laser diode side-pumped enfolded Nd∶YAG laser, a 27.3% optical-optical slope efficiency was obtained.