To prolong the gel time of polysilicic acid， the effect of various factors， including the pH value， the concentration of sodium ion in the industry water glass，kinds of acidifier，boron added，aluminium and iron ions introduced is observed respectively. The experimental results show that the stability of polysilicic acid is effected greatly by the pH value，and by reducing the concentration of sodium ion in pH value near neutral area and introducing acetic acid，B，Al and Fe，the stability of polysilicic acid can be enhanced.The applicable conditions about the effect factors are educed. The study of increasing the stability has been combined with the polymerization mechanism of polysilicic acid. The results can provide some theoretical basis for the studies and applications of inorganic compound flocculants.