Temporally and spatially resolved absorption spectrometry has been used to study molecular LaO absorption in laser ablated plume from an La2O3 target The absorption time-of-flight (TOF) spectra of ground-state LaO molecules were measured The TOF spectra indicate that only one component is observed in vacuum and in an Ar ambient, while there are two component, a fast and a broad slow component, observed at higher O2 pressure. The absorbance of LaO molecules decreases gradually with increasing ambient gas pressure. The dependence of the absorbance of LaO on the detection distance can be fitted by an exponential decay equation. The shock wave model is used to describe the behavior of ground state LaO molecules in high O2 pressure region.