This study presents an ultra-sensitive carboxyl-functionalized graphene oxide (GO-COOH)-based surface plasmon resonance (SPR) immunosensor using Carbohydrate antigen (CA) 199 (CA199) biomarker to detect pancreatic cancer. A study of the effect of carboxyl-GO-based biocompatible composites with protein on sensing adhesion and diagnostic engineering is an useful technique. The optical and sensing properties of carboxyl-GO sheets improved sensitivity and affinity for molecular detection of biomarker. In the experiment, the SPR biosensor with carboxyl modified graphene oxide as substrate was tested to detect the immune response of CA 199. The literature indicates that 79% of pancreatic cancer patients have carbohydrate antigen 19-9 content above 37 unit/ml. The experimental results show that the lowest antigen detection limit of surface plasmon resonance biochip with carboxyl modified graphene oxide as the substrate can reach 10 unit/ml.