Energy level regulation and crystallization optimization of materials are the key ideas to improve the performance of organic solar cells. In this work, based on clarifying small molecule structure, we introduce thiazole into different positions of π-bridge and investigate its effect on the properties of small molecule donors (SMDs) and related device performance. As thiazole gradually moves towards BDT, the electron clouds of these three SMDs (BTTTzR, BTTzTR and BTzTTR) tend to be divided on the framework, gradually deepened the energy level, and enlarged band gaps. In terms of device performance, the BTTzTR:Y6 device shows good inhibition of charge recombination and phase separation performance, and the efficiency of the related OSCs is 7.87 %. Morphological analysis shows that BTTTzR is prone to form nanoscale microcrystals after thermal annealing treatment, with slightly decreased performance of 6.79 %. BTzTTR is affected by difficult intramolecular charge transfer and weak intermolecular interactions, shows the worst performance of 5.77 %. This work is conducive to a deeper understanding of the electron-deficient substitution of the π-bridge in SMDs.