Background This study aimed to investigate the effect of nursing aids (NAs) and registered nurses (RNs) mixed nursing staffing model with different ratios on the nursing outcomes and cost in China. Methods Five thousand and ninety-one patients treated at Neurology and Neurosurgery Center were consecutively recruited in this study and divided into three groups according to the proportion of NAs and RNs: 100% RN group (N = 1756), 90% RN group (N = 1654), and 75% RN group (N = 1681). Nursing outcomes including medication error, unplanned endotracheal tube extubation, bloodstream infection, respiratory tract infection, urinary tract infection, pressure ulcer, ventilator weaning, morality, hospital stay, and nursing cost were recorded. Results No difference of patients' characteristics or RNs' characteristics among three groups was observed. Three-group comparison disclosed that medication error, urinary tract infection, ventilator weaning, and nursing cost were different among three groups, while no difference of unplanned endotracheal tube extubation, bloodstream infection, respiratory tract infection, pressure ulcer, mortality, or hospital stay was found. Two-group comparison revealed that medication error and nursing cost were reduced in 75% RN group compared with 90% RN group and 100% RN group, but urinary tract infection was increased while ventilator weaning was decreased in 75% RN group and 90% RN group compared with 100% RN group. Multivariate logistic regression also validated 75% RNs independently correlated with decreased medication error, ventilator weaning, and increased urinary tract infection. Conclusions NAs and RNs mixed nursing staffing model with 75% RNs reduces medication error and nursing cost, while increasing urinary tract infection and decreasing ventilator weaning.