Interpenetrating bulk-heterojunction structure with domain size of 10-20 nm is the ideal morphology for carrier generation, separation and transportation in organic solar cells. However, film morphology of p-DTS(FBTTh2)2/EP-PDI blend systems is always far from satisfactory for nano-structure of optimal devices. When the weight ratio of p-DTS(FBTTh2)2/EP-PDI is greater than 8:2 or smaller than 4:6, large phase separation structure is observed induced by p-DTS(FBTTh2)2 or EP-PDI crystallization. When the ratio of p-DTS(FBTTh2)2/EP-PDI changes from 7:3 to 5:5, no obvious phase separation can be detected. In order to obtained the interpenetrating structure with domain size of 10-20 nm, we tuned the molecule diffusion of p-DTS(FBTTh2)2 and EP-PDI by thermal annealing. When the annealing temperature is T < Tm EP-PDI + Ta or T > Tc p-DTS(FBTTh2)2 - Tb (T is thermal annealing temperature, Ta and Tb are constants), molecule diffusion rate of both p-DTS(FBTTh2)2 and EP-PDI are too slow or too fast, resulting in no phase separation or large phase separation morphology. When T is between Tm EP-PDI + Ta and Tc p-DTS(FBTTh2)2 - Tb, p-DTS(FBTTh2)2 crystallized and formed framework. Thus p-DTS(FBTTh2)2 crystallization inhibited massive crystallization of EP-PDI, leading to the formation of bi-continuous phase separation structure. As a result, we constructed the phase diagram according to different blend ratio and thermal annealing temperature of the p-DTS(FBTTh2)2/EP-PDI crystalline-crystalline small molecule blend system. Furthermore, we revealed the phase separation mechanism and summarized guideline to construct bi-continuous phase separation structure with proper domain size and phase purity. Based on the bi-continuous phase separation structure we got, high performance of 4.25% power conversion efficiency was obtained.