Walnut-shell activated carbons (WSACs) with high surface area and predominant micropore development were prepared by KOH chemical activation. The effects of carbonization temperature, activation temperature, and ratio of KOH to chars on the pore development of WSACs and PH3 adsorption performance of the modified walnut-shell activated carbons (MWSACs) were studied. Criteria for determining the optimum preparation conditions were pore development of WSACs and PH3 breakthrough adsorption capacity of MWSAC adsorbents. The result shows that the optimum preparation conditions are a carbonization temperature of 700 degrees C, an activation temperature of 700 degrees C, and a mass ratio of 3. The BET surface area and the micropore volume of the optimal WASC are 1636m(2)/g and 0.641cm(3)/g, respectively. The micropore volume percentage of WSAC plays an important role in PH3 adsorption when there is a slight difference in BET surface areas. High-surface-area WSACs with predominant micropores are suitable for PH3 adsorption removal. The MWSAC adsorbent owns the biggest PH3 breakthrough adsorption capacity (284.12mg/g) due to the biggest specific surface area, total pore volume, and micropore volume percentage. The MWSAC adsorbent will be a potential adsorbent for PH3 adsorption removal from yellow phosphorus tail gas.