Glutaminase (GLS1) is a cancer energy metabolism protein which plays a predominant role in cell growth and proliferation. Because of its major involvement in malignant tumor, small-molecule GLS1 inhibitors are urgently needed to assess its therapeutic potential and for probing their underlying biology function. Recent studies showed that targeting the allosteric binding site represented a promising strategy for identifying potent and selective GLS1 inhibitors. Herein, we present the synthesis of two fluorescent probes targeting the allosteric binding site of GLS1 and their usage as mechanistic tools in multiple applicable assay platform. The fluorescence polarization (FP)-based binding assay enables easy, fast, and reliable screen of allosteric inhibitors from our in-house compound library obtained through click chemistry method. The obtained compound C147 (named as CPU-L1) has been proved to be more potent and with greater solubility than the control compound CB839, which could serve as promising leads for further optimization as novel GLS1 inhibitors.