The time evolution of the ground state of a one-dimensional hydrogen atom in the intense laser pulse is studied directly in momentum space. A finite matrix representation of operators is developed to solve nonperturbatively the time-dependent Schrödinger equation. We find that some numerical limitations in coordinate space of above-threshold ionization (ATI) can be overcome, and clear physical interpretations of the numerical data are possible. Both the ATI and harmonic-generation spectra are obtained. The correlations between these two multiphoton phenomena are discussed.