The multiphoton excitation of NH3 in laser fields is discussed using the second quantization and unitary transformations.The stretch-vibration energy spectrum obtained from an optimized Hamiltonian is in good agreement with that of experiments.The changes in transition probabilities with the frequencies of laser fields and the lime-dependent transition probabilities at a field intensity of 1.1×10-8 W/cm2 are presented.Finally,the energy absorption is calculated. The multiphoton excitation of NH3 in laser fields is discussed using the second quantization and unitary transformations. The stretch-vibration energy spectrum obtained from an optimized Hamiltonian is in good agreement with that of experiments. The changes in transition probabilities with the frequencies of laser fields and the lime-dependent transition probabilities at a field intensity of 1.1 × 10 -8 W / cm 2 are presented. Notably, the energy absorption is calculated.