Objective To investigate the effect of ultrasound-guided iliac fascial space block and unilateral spinal anesthesia in posterior hip arthroplasty for elderly femoral neck fractures. Methods Fifty elderly patients with femoral neck fractures undergoing posterior hip arthroplasty in the Department of Anesthesiology, Yangjiang Hospital of Traditional Chinese Medicine from January to July 2021 were selected and divided into a control group and an observation group, with 25 cases in each group. The control group was treated with combined spinal-epidural anesthesia, and the observation group was treated with ultrasound-guided iliac fascial space block combined with unilateral spinal anesthesia. Anesthesia effects, blood pressure fluctuations, sensory and motor-related time, and postoperative analgesic pump usage, pain, and adverse reactions were compared between the two groups. Results The excellent and good rate of anesthesia in the observation group was higher than that in the control group, and the difference was statistically significant （P<0.05）; the excellent and good rate of blood pressure fluctuation in the observation group was higher than that in the control group, and the difference was statistically significant （P<0.05）; the sensory block time, motor block time, sensory recovery time, and motor recovery time were shorter in the observations group than in the control group, and the differences were statistically significant （P<0.05）; the observation group first pressed the analgesic pump later than the control group, pressed less times of analgesic pumps within 48 h after operation, and conducted less number of remedial analgesia, and the differences were statistically significant （P<0.05）; the observation group showed lower at-rest Visual Analog Scale （VAS） scores at postoperative 4 h, 8 h, and 24 h, as well as during-movement VAS scores at postoperative 12 h and 24 h than the control group, the differences were statistically significant （P<0.05）; the difference in the incidence of adverse reactions between the two groups was not statistically significant （P>0.05）. Conclusion Ultrasound-guided iliac fascial space block plus unilateral spinal anesthesia shows a better anesthesia effect than combined spinal-epidural anesthesia during posterior hip arthroplasty for elderly femoral neck fractures, which can shorten sensory and motor-related time and relieve postoperative pain, with little influence on patients’ blood pressure, and induce few adverse reactions.