The viscoelastic behavior of sodium thiocyanate concentrated solution of graft copolymer of acrylonitrile onto casein (AN-g-casein) was investigated in detail by nitrogen pressure capillary rheometer. The experimental results show that the concentrated solution of AN-g-casein is a non-Newtonian fluid of shear thinning. The end correction increases with the increase of shear rate. In the course of practical spinning, the real shear stress only attributes about 35% to the apparent shear stress, which confirms the essentiality of end correction for the capillary extrusion of the concentrated solution of the graft copolymer. By using the Dewitt model, the Couette correction and recoverable shear strain S-R were analyzed and separated. Quantitative function relation of xi and S-R versus shear rate and temperature were deduced. It is a common equation within the range of experiment. In practical spinning process, the real viscoelastic parameters can be easily predicted by only a piece of flowing curve based on the common equation. Real elastic model G under any shear rate in the range of experiment was calculated. It was found that G increases with an increase of shear rate, which is opposite to the conclusion reported by Hayahara. The main reason for the error may be that Hayahara used the same Couette correction at a higher shear rate. Further, the die swell in the spinning process can be estimated efficiently. The theoretical base for the stability of AN-g-casein spinning technology was provided.