Composite microcapsules of sodium alginate/urea-formaldehyde resin were prepared in two steps. Then sodium alginate(SA) was added to the condensation polymerization of urea-formaldehyde resin(UF) , forming composite shells at the second step. FTIR, TG, SEM and granulometer were used to characterize the structure, composition, morphology and size distribution of microcapsules. The results showed that the agglomeration could be effectively improved and the encapsulation efficiency was increased to more than 60%. In addition, the composite microcapsules have higher release ability than UF microcapsules as the cumulative release was increased from 27.5% to 44.5% within 72 days. Moreover, the drug release behavior of modified microcapsules exhibit some pH and temperature dependence. More than 50% of the avermectin added was released within 27 days in acidic medium at 25℃. The release behavior generally followed the Korsmeyer-Pappas dynamic model.