Abstract: On the basis of Rigrod’S theory,we have developed a steady-state theory of unstableresonator for excimer laser.The small-Signal gain g₀,unsaturable absorption coefficient αandsaturable intensity I_s are estimated by measuring the output intensity of parallel-plane oscillatorswith seven output couplings under different pump conditions. Thus,the output intensity I_out of positive-branch confocal unstable resonators as a function of magnification M are obtained throughunstable resonator steady-state theory.The calculated parameters of unstable resonator are inagreement with the experimental results.