Small carbon cluster anions doped with single selenium atom CnSe- (n = 1 to 11) were produced by laser ablating the mixture of selenium and carbon powders and were recorded by mass spectrometry. Distribution of the doped cluster ions was found to vary with the mixed ratios of the sample, but only the cluster with even number of carbon atoms could be produced in the experiment. Experiment of collision-induced dissociation (CID) verified molecular formulas of the selenium polycarbon clusters and found that they tend to lose a Se atom (for smaller size) or a CSe unit (for larger size). To examine the odd/even alternation effect and other structural features, the duster ions were further investigated by ab initio calculations with ROHF and B3LYP methods using 6-31G* basis set. Based on the studies of other CnX- clusters and statistical distribution of the CnSe- clusters, geometry of the clusters was assumed to be linear chain with the selenium atom locating at its terminal and was fully optimized in the calculation. The calculated total energy, vertical electron detachment energy and fragmentation energy, bond length, and other structural parameters exhibit the alternation effect, showing that CnSe-clusters with even n are more stable than the odd clusters, in a good consistency with the mass spectrometric observation. The structural difference between opposite parties of CnSe- is found to reduce following the increasing number of carbon atoms. The theoretical investigation also shows that electron correlation has to be considered in the calculation and the result obtained by the density function method is sufficient to describe the structural features of CnSe- clusters. Besides, dissociation energies of six dissociation channels, losing C, C-2, C-3. CSe, C2Se, or Se fragment, were calculated for CnSe-anions and the result also exhibits the parity effect and matches well with the CID experimental observation.