We present oscillator strength spectra of Rydberg lithium atoms in parallel electric and magnetic fields obtained via the B-spline basis set method combined with model potential. Both the radial and angular wavefunctions are expanded in terms of B-spline functions. The results are in good agreement with previous high-precision experiments. By analyzing the components of partial-wave, we give an explanation to the oscillator strength of part of the spectrum. The method of this paper is simple and effective. It is suggested that the present method can be applied to deal with the problem of Rydberg atom in crossed magnetic and electric field.