Abstract: Objective To investigate the serum complement C5 a level and its relationship with inflammatory factors in patients with Alzheimer's disease(AD) in order to elucidate its clinical significance. Methods Total 110 patients with AD in Hangzhou Seventh People's Hospital from January to December 2018 were selected as AD group, and 110 healthy subjects were selected as control group. The serum C5 a level, high sensitive C-reaction protein(hs-CRP), interleukin-1β(IL-1β), IL-6 and tumor necrosis factor-α(TNF-α) levels were measured. Results The score of the simple mental state scale(MMSE) of the AD group[(15.62±3.21) scores] was lower than that of the control group[(29.34±3.82) scores, t=28.839, P<0.05], and the level of C5 a[(73.51±9.42) pg/mL] was higher than that of the control group[(64.29±9.34) pg/mL, t=7.290, P<0.05]. The levels of serum hs-CRP, IL-1β, IL-6 and TNF-α in the AD group[(5.03±0.60) pg/mL,(37.49±7.46) pg/mL,(105.34±13.13) pg/mL,(196.54±12.06) pg/mL] were higher than those in the control group(t=45.395, 9.939, 25.704, 7.086, all P<0.05). The levels of serum C5 a, hs-CRP, IL-1β, IL-6 and TNF-α in AD patients were negatively correlated with MMSE scores(r=-0.421,-0.523,-0.436,-0.445,-0.507, all P<0.05). The serum C5 a was positively correlated with serum hs-CRP, IL-1β, IL-6 and TNF-α levels in AD patients(r=0.564, 0.602, 0.641, 0.539, all P<0.05). Conclusion Serum complement C5 a level is elevated in patients with AD, and it is closely related to cognitive function and serum inflammatory factor levels in AD patients.