Objective：To investigate the clinical value of small pulmonary vessels parameters measured by chest CT in evaluating the classification of different types of pulmonary hypertension（PH）and the severity grading of connective tissue disease s - related pulmonary hypertension（CTD-PH）. Methods：A retrospective analysis included 170 PH patients，including 60 CTD-PH patients，52 idiopathic PH（IPAH）patients，and 58 chronic obstructive pulmonary disease - related PH（COPD - PH）patients，with 120 healthy controls（HC）included as controls. The ratio of the sum of the cross-sectional area（CSA）of small pulmonary vessels with CSA ＜5 mm² （%CSA_＜5）and between 5 to 10 mm²（ %CSA_{5- 10}）to the total CSA of the lung measured by chest CT was compared among the four groups using one-way ANOVA or Kruskal-Wallis test，followed by pairwise comparisons. Receiver operating characteristic（ROC）curve analysis was used to evaluate the performance of %CSA for differentiating mild to moderate CTD-PH（CTD-LM-PH）from severe CTD-PH（CTD-S-PH）patients，and calculate the area under the curve（AUC），sensitivity and specificity. Results：The %CSA_＜5 of the IPAH and COPD -PH groups were significantly lower compared to the HC group（P ＜ 0.001）. Additionally，the %CSA_{5 - 10} of the COPD -PH group showed a significant decline compared to the HC group（P=0.038），whereas the %CSA5-10 of the CTD-PH and IPAH groups was significantly high compared to the HC group（both P ＜ 0.05）. In comparisons between different types of PH groups，the %CSA_＜5 and % CSA_{5 - 10} of the CTD - PH group were higher than those of the COPD - PH group（P ＜ 0.001）. The % CSA_{5 - 10} of the IPAH group was significantly higher than that of the CTD -PH group（P=0.022），while there was no significant difference in the %CSA_＜5 between the IPAH and COPD-PH groups（P=0.833）. The %CSA_＜5 of CTD-S-PH group was significantly lower than that of CTD-LM-PH group（P= 0.004）. The ROC curve analysis showed that the optimal cutoff value for %CSA_＜5 to predict CTD -S -PH was 0.804，with an AUC of 0.710（95%CI：0.573-0.847），sensitivity of 0.714 and specificity of 0.320. Conclusion：The quantitative parameter %CSA assessed by chest CT can distinguish different types of PH. The %CSA_＜5 can serve as a reference for evaluating the severity of CTD-PH.