Nitrate, as one of the important nutrients in seawater, influences the constant ratio of nitrogen to phosphorus, which is closely related to phytoplankton survival. In this work, a Cu-nanosphere-modified gold microwire electrode was used as the working electrode for determining nitrate in an artificial seawater sample with salinity of 35 parts per thousand by a differential pulse voltammetry technique. Under the optimized conditions, the detection linear range is from 1 mu M to 2000 mu M, the limit of detection is 0.33 mu M, and the response time for a single sample is 5 min. Then, to reduce the influence of factors such as temperature, humidity, and microbial environment during sample transporting on the nitrate concentration in real seawater, a portable electrochemical system was introduced for on-site detection. Rapid field determination results show that nitrate levels correlate with tides, proving the portable system's reliability.