In this stady, a graphene-chitosan (GR-CS) modified glassy carbon electrode (GCE) was prepared by in situ reduction. The obtained electrode was characterized by FT-IR spectroscopy and Raman spectroscopy, and the results showed that the graphene oxide (GO) was deoxygenized successfully. On this basis, the electrochemical behavior of 4-nonylphenol (4-NP) was studied by using cyclic voltammetry and differential pulse voltammetry. It was found that the oxidation current on GR-CS/GCE electrode was significantly larger than on the GCE and GO-CS/GCE electrodes. The oxidation potential shifted negatively, indicating that the GR-CS/GCE electrode had obviously catalytic activity on the 4-NP's oxidation. Experimental parameters, such as the accumulation potential and time, scan rate, and pH value of buffer solution were optimized. Under the optimized conditions, the oxidation peak current was proportional to 4-NP concentration in the range between 0.01–40.0 μM, the linear equation was I (μA) = 0.364C (μM) + 0.618 (R = 0.9988), and the detection limit was 0.0052 μM (S/N = 3). Finally, this proposed sensor was successfully employed to determine 4-NP in real samples with the recoveries ranging in 95.0%–101.0%.