A quasi-dynamics method is developed to search for cluster isomers and can quickly produce isomers which are probable in gas-phase growth under common experimental conditions. We studied the spectrum of C21 isomers by the method and performed molecular dynamics simulations on the clustering processes of 21 isolated carbon atoms in helium buffer, showing that the isomers formed in the dynamical process at higher probabilities are all contained in the isomer spectrum obtained by the method. The potential energy of the most stable configuration in our spectrum is much lower than that obtained by the genetic algorithm ［Chem. Phys. Lett. 364 213，2002］.