The new-domain nucleation by applying a weak electric field is an important process for the ferroelectric domain inversion in ferroelectrics. We studied the process of the new-domain nucleation in ferroelectrics using the classical model, and found that the rate of nucleation in the electric field did not agree with the experimental result. Based on Tagantsevs model, we improved the classical model of the new-domain nucleation using a long flat dagger model instead of the ellipse model. Using the improved model, the result of the theoretical calculation agreed well with the experimental measurement.