The energy spectrum and oscillator strength of hydrogen atom in static magnetic field have been calculated using basis set method. Both the radial and angular wave-functions are expanded in terms of B-spline functions. The resultant size of Hamiltonian matrix is reduced considerably, which enables the calculations to be carried out in personal computer. Diamagnetic spectrum of Rydberg hydrogen atom from -40cm-1 to the field free ionization threshold was obtained with high accuracy. The results have been compared with those of most accurate ones in the literature. The method presented in this paper can be extended easily to the region with positive energy and to the cases of non-hydrogen atoms and the crossed field problem.