The invention relates to an electrochemical sensor for determining nitrogen content, comprising: a solid electrolyte layer, having MgO-doped zirconia MgSZ as a matrix material; and an auxiliary electrode layer, which replaces nitrogen-doped MgO-stabilized zirconia formed by some oxygen ions in MgSZ with nitrogen ions, that is, MgNSZ is the matrix material. The invention further comprises a methodfor preparing the electrochemical sensor, comprising the step of forming a MgNSZ auxiliary electrode layer of a predetermined thickness on the surface of the MgSZ solid electrolyte by using a laser surface nitriding treatment technology on the surface of the prepared MgSZ solid electrolyte, which is not only simple in process with dense and non-porous MgNSZ auxiliary electrode layer, but also firmly bonded to the matrix of the MgSZ solid electrolyte to obtain good compatibility and small interface resistance. Therefore, the preparation method can be used to advance the nitrogen sensor with excellent performance.