The non-conductive substrate is often metallized through electroless plating method. Prior to the electroless plating, the substrate surfaces need to be firstly Pd activation pre-treated. The traditional "two-step" activation process, i.e., sensitization-activation, has been gradually obsoleted because of poor controllability and uniformity. A "one-step" activation process using Pd colloid has been widely used in industry, especially for the microvia metallization treatment in printed circuit board (PCB) fabrication. The bottleneck problem of this technology is the preparation of the Pd colloid solution with high concentration and excellent catalytic activity. The aim of this work is to develop a preparation method of the Pd colloid with high concentration and high quality. Pd colloid was prepared by a continuous reduction reaction with minor content. By mean of this process, the Pd concentration of the prepared colloid can exceed 2%. The morphology, microstructure and composition of the Pd colloid were characterized by SEM, TEM, XRD and XPS, respectively. The activate ability of the Pd colloid was examined by electroless Cu and electrochemical test. It was found that the average diameter of the Pd particles was less than 4 nm. Even if the concentration of Pd was less than 25 mg/L, this Pd colloid still had good activation ability for electro less Cu. The result demonstrated that the shell structure of the Pd micelle played a key role for the activation ability. The shell of Pd micelle was consisted of Sn²⁺, Sn⁴⁺ and Cl^-, and generally formed two structures, [PdSn2]Cl₆ and [PdSn3]Cl₈. For the structure of [PdSn3]Cl₈, the failure of the hydrolysis could lead to the loss of activation. The preparation method in this work can effectively avoid the occurrence of [PdSn3]Cl₈, which greatly improved the activation ability of the Pd colloid.