By analyzing the effect of the PdCl2 concentration, Sn / Pd (molar ratio), reaction temperature, and NaCl concentration on the activity and stability of colloidal palladium, the optimal preparation conditions of salt-based colloid palladium of low concentration for the PCB electroless plating was obtained that the concentrations of PdCl2 and NaCl were 0.4g/L and 160g/L respectively, mole ration of Sn/ Pd was 30:1, reaction temperature was 60±2℃. Four different additives (tin sodium, urea, vanillin, and ascorbic acid) were added into the activation solution obtained according to the above conditions respectively. The effect of four different additives on the activity and stability of activation solution was investigated. The obtained results showed that the urea had a good effect on improving the activity and stability of the activation solution. The suitable minimum concentration of urea was 10 ~ 15g / L.