Objective To investigate the anesthetic effect of subarachnoid anesthesia of light specific gravity bupivacaine combined with spinal anesthesia and its effect on circulatory and respiratory systems in the elderly patients undergoing proximal femoral nail antirotation (PFNA). Methods Sixty elderly patients who underwent PFNA surgery in our hospital from August 2015 to August 2018 were selected as subjects. They were randomly divided into 2 groups:light specific gravity group and heavy specific gravity group, with 30 cases in each group. Distilled water solution (6ml) containing 0.25% bupivacaine was administered in the former group, and glucose solution (3ml) containing 0.5% bupivacaine was administered in the latter group. The sedation levels and changes in circulatory and respiratory systems were detected and compared between the 2 groups before anesthesia (T₀), after anesthesia plane fixation (T₁), and in 5(T₂), 15(T₃), 30(T₄) and 60min (T₅) after administration. Patients′ cognitive status was assessed with mini-mental state examination (MMSE) scale. The anesthetic effects and adverse reactions of the 2 groups were compared. Data processing was performed using SPSS statistics 20.0. Results The Ramsay scores at onset time of anesthesia and the T₀-T₅ time points were almost similar between the 2 groups (P>0.05). The light specific gravity group had lower [JP+1]percentages of the patients with anesthesia blockplane ≥ T₁₀ (20.00% vs 43.33%), and reduced usages of ephedrine (3.33% vs 43.33%) and atropine (6.67% vs 36.67%) when compared with the heavy specific gravity group (P<0.05). Compared with the T₀ time point, the mean arterial pressure (MAP), heart rate, systolic pressure, diastolic pressure, respiratory rate, and pulse oximetry (SpO₂) of the 2 groups were in a trend of decrease at the T₂-T₅ time points. MAP and SpO₂ were significantly lower in T₁-T₅ time points, and heart rate, systolic blood pressure, diastolic blood pressure and respiratory rate were also obviously lower at T₂-T₅ time points in the heavy than the light specific gravity group (P<0.05). The MMSE score at 24h after anesthesia was significantly higher in the light than the heavy specific gravity group [(24.14±2.14) vs (26.83±2.34), P<0.05]. The incidences of cognitive dysfunction (3.33% vs 16.67%) and adverse reactions (23.33% vs 46.67%) in the light specific gravity group were significantly lower than those of heavy specific gravity group (P<0.05). Conclusion Satisfactory anesthetic effect is achieved in the elderly patients undergoing PFNA with combination of subarachnoid anesthesia of light specific gravity bupivacaine and spinal anesthesia. It has good safety and less effect on the circulatory and respiratory systems, and is worthy of further application in clinical practice.