The beam quality factor of parametric light from KTP optical parametric oscillator pumped by a doubling frequency Nd∶YAG laser (532 nm) was measured by means of laser beam analyzer. The beam quality dependence of different pumping conditions is discussed such as pump power, pump beam diameter, OPO cavity length, phase matching mode, and pumping with or without reflection. In the theoretical analysis, double refractive walk off term is introduced into coupling wave equations which are numerically solved with light propagation method and Rungue Kutta algorithm. The molding results agree well with the observed OPO performance.