A new simple method to measure the spatial distribution of the electric field in a dust particle trap is proposed. The method is based on the experimental investigation of vertical oscillations of a single dust particle trapped in a standing striation. The excitation of the nonlinear oscillations with the large amplitude of the order of 1.5 mm is due to the periodic movement of the standing striation caused by the squarewave modulation of the discharge current. The amplitude frequency characteristics are investigated depending on value of the modulation depth. The theory of the anharmonic oscillator provides a good quantitative description of the experimental data. The potential energy curve is calculated using the values of anharmonic coefficients.