The egg sac is the place of spider laying eggs and breeding offspring for the protection of future generations. Argiope bruennichi, a common forestry spider, generally produces eggs 3-6 times and weaves 3-6 egg cases. In this program, the weight, biomass of egg sacs and egg sac numbers of A.bruennichi fed at room temperature are studied. The results show that increased biomass decreased with the increase of the number of spawning egg sacs, but the input for egg sac increased successively; the input of the egg sac silk has also increased with the increase of spider invest for egg mass, which puts more energy for offspring; the number of eggs of per egg mass increased, and per egg weight reduced with the increase of the numbers of spawning egg sacs.