Laser desorption supersonic jet spectroscopy, which can enable the conformer-selected measurements, was improved by optimizing the matrix substances and introducing a high-pressure supersonic jet expansion to obtain high desorption efficiency and high jet-cooling effect. The improved laser desorption supersonic jet technique was applied to catecholamine neurotransmitters and their relevant molecules. By using resonance enhanced multiphoton ionization and UV-UV hole burning spectroscopies, the number of conformers of each molecule was determined. By comparing them, we found a propensity rule, named as “doubling rule”, in the conformational evolution of relevant molecules. However, the numbers of conformers of catecholamines are much less than the expected ones by the doubling rule. Thus we concluded that the numbers of conformers of catecholamines are specifically small. We discussed the reason of decreasing the number of conformers of catecholamines, and which was interpreted as due to the specific flexibility of catecholamines.