Objective To observe the change characteristics of C4d， complements C3 and C4 in patients with systemic lupus erythematosus and lupus nephritis （SLE-LN）， and to analyze their clinical value in the early diagnosis of SLE-LN， the evaluation of disease severity， and the prognosis prediction. Methods A total of 98 patients with systemic lupus erythematosus （SLE） who received treatment at the Affiliated Hospital of Chengde Medical University were recruited as the research subjects from January 2024 to January 2025. Based on the occurrence of lupus nephritis （LN）， they were divided into the SLE-LN group （38 cases） and the simple SLE group （60 cases）. The levels of plasma C4d， complements C3， and C4 were measured using the enzyme-linked immunosorbent assay （ELISA）. The 24-hour urine protein， serum creatinine （Scr）， blood urea nitrogen （BUN）， and uric acid （UA） were determined， and the estimated glomerular filtration rate （eGFR） was calculated. The correlation between C4d， C3， C4 and 24-hour urine protein， as well as renal function， was analyzed by Pearson correlation analysis. All 98 SLE patients were followed up for 12 months. According to the changes in renal function， they were divided into the poor prognosis group （with deteriorated renal function， 29 cases） and the good prognosis group （with stable renal function， 66 cases）. The predictive value of C4d， C3， and C4 for the prognosis of SLE was analyzed by receiver operating characteristic （ROC） curves analysis. Results The plasma C4d level in the SLE-LN group was significantly higher than that in the simple SLE group， whereas the C3 and C4 levels were significantly lower than those in the simple SLE group （P < 0.05）. The 24 - hour urine protein， Scr， BUN， and uric acid （UA） levels in the SLE-LN group were significantly higher than those in the simple SLE group， while the eGFR was significantly lower than that in the simple SLE group （P < 0.05）. Pearson correlation analysis indicated that the C4d level was significantly positively correlated with the 24-hour urine protein， Scr， BUN， and UA levels （P < 0.05）， and significantly negatively correlated with the eGFR （P < 0.05）. The C3 and C4 levels were significantly negatively correlated with the 24 - hour urine protein， Scr， BUN， and UA levels （P < 0.05）， and significantly positively correlated with the eGFR （P < 0.05）. The plasma C4d level in the poor prognosis group was significantly higher than that in the good prognosis group， while the C3 and C4 levels were significantly lower than those in the good prognosis group （P < 0.05）. Receiver operating characteristic （ROC） curve analysis showed that the area under the curve （AUC）， sensitivity， and specificity of the combined detection were 0.868， 79.31%， and 86.36% respectively， which had the highest predictive efficiency for the prognosis of SLE patients. Conclusions Compared with patients with simple SLE， the level of plasma C4d is higher， while the levels of complements C3 and C4 are lower in patients with SLE-LN. All three of these indexes are significantly correlated with 24-hour urine protein and renal function. The combined detection of plasma C4d， C3， and C4 has a higher predictive efficiency for the prognosis of SLE patients.