A folded 3-mirror laser resonator in which a Cr4+∶YAG crystal is employed as a saturable absorber for passively Q-switching and a Brewster plate to compensate astigmatism was designed. The resonator has small radius of laser waists, so it is very suitable to the laser diode pumped intercavity frequency-doubling lasers with quasi-phase-matched frequency-doubler. The expressions to calculate the radius and the place of laser waist, curvature of the mirrors used in the resonator, the thickness of a Brewster plate, and folding angle of the resonator were presented. The performance of continous-wave laser diode pumped passively Q-switched PPLN intercavity frequency-doubling laser with the folded 3-mirror laser resonator was investigated. As the result that both fundamental and second harmonic waves have the same polarization, no green-noise problem exists. The pulse series of laser output are stable. The laser has the average output power 42.7 mW under the CW input power 500 mW.