Objective: To optimize the technology and process for recovering immune cells from discarded leukocyte filter through reverse flushing, and to investigate the effects of different reverse flushing solutions and physical conditions (temperature, soaking, and ultrasound) on the cell recovery rate. Additionally, to evaluate the feasibility and quality of isolating and culturing NK cells from the recovered cells. Methods: Normal saline, PBS, and PBS-EDTA were used as reverse flushing solutions to backwash the filter, and the white blood cell recovery rates were compared. The reverse flushing efficiency was optimized by adjusting the temperature of the reverse flushing solution, filter soaking treatment and ultrasonic treatment. The Ficoll separation method was employed to isolate peripheral blood mononuclear cells (PBMCs) from the elution solution and conduct in vitro expansion and culture. Lymphocyte subsets were identified and quantified using flow cytometry. Results: The physiological saline served as the backflush fluid was the most stable. The reverse temperature had a significant impact on the recovery rate of white blood cell subgroups. The recovery rates of lymphocytes and monocytes at 37℃ were significantly higher than those at 4℃ [(35.56±5.55)% vs (22.88±0.31)%, (32.42±4.82)% vs (8.00±1.6)%, t=5.581 and 5.758, respectively, both P<0.001)], and at 25℃ [(35.56±5.55)% vs (22.42±6.13)%, t=5.904, P<0.001; (32.4±4.82)% vs (21.11±1.23)%, t=2.629, P=0.030)]. The recovery rates of white blood cell subgroups after 37℃ immersion were higher than those after direct reverse flow at 37℃, but the difference was not statistically significant (P>0.05). The recovery rate of cells after ultrasonic immersion at 25℃ was further increased compared to that after immersion at 25℃, but the difference was not statistically significant (P>0.05). The purity of the NK cells obtained through sorting reached (94.2±4.5)%, and the viability was (94.8±0.9)%. After 15 days of culture, the cell count was expanded approximately 450 times, yielding more than 10 /mL highly active NK cells. Conclusion: By optimizing the backflushing process, immune cells can be efficiently recovered from discarded filters. The 37℃ backflow liquid has the best backflow effect; soaking and ultrasonic treatment can be used as auxiliary methods to improve recovery rates. The NK cells from this source have high purity, good activity, and strong expansion ability, and can be used as a reliable cell source for immunotherapy research, and in line with the circular economy concept of resource reuse.