Abstract: Objective To study the diagnostic value of serum IgG4 content and ratio of IgG4 to total IgG in IgG4 related diseases (IgG4-RD). Methods Taking serum IgG4≥1.35g/L as the upper limit of reference value, the positive rates of IgG4 in the control group, other autoimmune diseases group and IgG4 RD group were 1.25%, 16.67% and 100% respectively. There was significant difference in the positive rate between the IgG4 RD group and the control group (Χ2= 108.306, P<0.05). The positive rate of other autoimmune diseases group was significantly different from that of the control group (Χ2=12.368, P <0.05). Compared with the control group, the serum IgG, IgG4 and IgG4/IgG ratio in IgG4 RD group were significantly increased (P<0.05), while the serum IgG and IgG4 in other autoimmune diseases group were significantly increased (P<0.05), but the IgG4/IgG ratio was not significantly increased (P>0.05). When serum IgG4≥2.49g/L combined with IgG4/IgG≥13.44% was used as the judgment standard, the positive rate of serum IgG4 in other autoimmune diseases decreased from 16.67% to 3.33%, and the diagnostic specificity was significantly improved. Conclusion The upper limit of the reference value of IgG4 level is 1.35g/L, which is not specific in the diagnosis of IgG4-RD. The combined application of serum IgG4≥2.49g/L and IgG4/IgG≥13.44% has high diagnostic efficiency in the diagnosis of IgG4-RD.