To investigate the effect of the ion source structure on the formation of doped nitrogen clusters with high nitrogen content, the conventional laser ablation ion source was improved by adding liquid nitrogen cooling and reducing the distance between nozzle of pulse valve and laser ablation spot, and nitrogen clusters doped by alkali metals and first transition metals were studied. LiN_n⁺ (n=6,8,10,12,14,16,18), Li₂N_n⁺ (n=8,10,12), NaN_n⁺ (n=2~27), VN_n⁺ (n=6,8,9,10,11,13,15), V₂N_n⁺ (n=17,19,21), CrN_n⁺ (n=2,4,6,8,9,11) and Cr₂N_n⁺ (n=9,10,11) were observed by time of flight mass spectrometer (TOF-MS). Results indicate that the improved ion source can significantly increase the number of generated doped nitrogen clusters and their nitrogen content. In addition, the type of doped metals has a great influence on the composition of generated clusters. In each type of metal-doped nitrogen clusters, the most abundant clusters are LiN₈⁺, NaN₁₂⁺, VN₁₁⁺ and CrN₈⁺, respectively.