Resonator parameters and thermo-stability conditions for plane-plane resonator o f multi-rod series connection solid-state lasers were investigated by means of m atrix optics.Stability areas of six-rod-connection solid-state lasers in a plane -plane resonator were calculated.The results show that stability areas and non-s tability areas of the multi-rod series-connection solid-state lasers distribute alternately.If the length of the resonator,the number(n) of laser rods and the s pacing (L) between two neighbouring laser rods are given and a symmetry configur ation (L1=L2) is adopted,then every stability area reaches a largest extent. If the condition (L1=L2=L/2) is supposed in plane-plane re sonator and the th ermo focus length (f) is greater than a lower limit value (fm),the st ability c ondition (01G2m),the resonator come into a deep non-stability area.The lower limit va lue (fm) is determined by the number (n) of laser rods and the spacin g (L) between two rods,the formula is fm=0.25L(if 2≤n≤10).