A four mirror folded cavity used for intracavity frequency doubled Nd:YAG laser was numerically analyzed in detail. It is shown that the large volume fundamental mode with a suitable stable region width can be achieved if proper cavity parameters are chosen, and the beam inside the cavity will make the frequency conversion efficient. Experimental results verify the predictions. The maximum output power is over 32 W, and the output beam quality, compared with that of the two mirror resonator, has been improved greatly.