The parametric sensitivity and optimal catalyst bed length of triple and quadruple fixed-bed catalytic reactors (FBCRs) are calculated using a two-dimensional pseudohomogeneous model for the partial oxidation of o-xylene to phthalic anhydride. The safety operation ranges of triple and quadruple FBCRs from various operating condition changes like initial concentration of the reactant, temperature of the cooling medium, and reactant and coolant flow rate are presented. Triple and quadruple FBCRs showed the behavior of wide operating range than single and double FBCRs on the initial concentration, coolant temperature, and reactant and coolant flow rate. Triple and quadruple FBCRs with nonuniform activities could assure safety operating condition and production increase by minute variation of operating condition. In order to design the FBCRs which could obtained maximum yield, we investigated the performance of FBCRs from catalyst bed length changes. Triple and quadruple FBCRs showed the best performance at L1=1.1, L2=1.0 and L3=0.48m, and L1=1.0, L2=0.5, L3=0.48 and L4=0.6m in case of total catalyst bed length z=2.58m.