By the improved suspending drop instrument, the surface tension of precursor polycarbosilane(PCS)is measured besides the influence mechanism of surface tension on the profile-degree of trilobal section PCS fiber is discussed. Through research by SEM, we discovered that the surface tension influences greatly on the formation of the profile-degree of trilobal section PCS fiber，the competitive action between it and the agglutination resistance decides the size of the profile-degree of trilobal section PCS fiber. We also discovered that the relation between the surface tension of PCS and the temperature is basically linear.The higher the temperature，the smaller the surface tension of PCS.