Taking the volcanic-intrusive complex in the Yanbei porphyry tin ore district as an example and based on the covariance plots of Ce, Nd, Sm, Eu, Yb in six different element-element,ratio-ratio coordinates, the authors have reestablished the petrogenic mechanism of the volcanic-intrusive complex in this district. The results suggest that the melt of the first acidic magmatic cycle was formed as a result of accumulative melting of basement rocks in South China(F=0.3). A part of acidic magma of the first cycle which was not extruded onto the surface might be crystallized in fractional crystallization state at depth,leading to the formation of residual granitic melts(F=0.2).