High aspect ratio β-MnO2 nanowires have been synthesized based on a facile and green technique without any chemical additive. The precursor solution of Mn3O4 nanocrystals was first synthesized by laser ablation of a manganese target in deionized water. Due to the high reactive and fresh surface of Mn3O4 nanocrystals produced by laser ablation in liquid, these nanocrystals were spontaneously assembled into the γ-MnOOH nanowires in the precursor solution after aging at room temperature. The high aspect ratio β-MnO2 nanowires were finally produced by the γ-MnOOH nanowires annealing at 300 °C for 3 h. For the high aspect ratio β-MnO2 nanowires, the high specific surface area is advantaged for gas absorption and the unique tunnel structure is good for gas molecule trapping. A gas sensor was made from the β-MnO2 nanowires for explosive gases. The investigations were carried out for the detection of various concentration of H2 at different temperatures, and the results demonstrated that the fabricated gas sensor can detect H2 down to 20 ppm with the sensitivity of 0.5 at 300 °C and short response time of 10s. For sensing CO and ethanol, the detecting concentration reached to 20 ppm at an operation temperature of 150 and 250 °C, respectively. These results can be comparable to that of the current advanced gas sensors made from metal oxide such as ZnO and SnO2, which showing that high aspect ratio β-MnO2 nanowires can be regarded as desirable candidate materials for fabricating gas sensors.