Downward continuation is one of the key steps in the processing of gravimetric and geomagnetic data. However, downward continuation is a typical ill-posed problem, and its computation is unstable. Therefore, the regularization methods are needed in order to realize the effective continuation of gravimetric and geomagnetic data, and the determination of regularization parameter is the most important content in the study of downward continuation by regularization method. According to the Poisson integral plane approximate relationship between observation and continuation data, and combining with Fast Fourier Transform (FFT) algorithm, the computation formulae were transformed to frequency domain so as to accelerate the computational speed. The Landweber regularization iteration method was introduced so that the instability could be overcome and the results precision could be improved, based on that the determination method of optimal regularization parameter in downward continuation was studied by L-curve method. The availability of regularization parameter was validated by simulated geomagnetic data, and continuation results in good precision were also derived.