Excess suspended particulate matter (PM) in constructed wetland (CW) substrates may reduce the substrate porosity and thus affect pollutant removal performance. However, it remains unclear how different PMs affect the nitrogen removal performance in CWs. In this study, kaolin and polystyrene (PS) were selected as two model PMs added to CW substrates at a concentration of 100 mg/L. Four CWs were constructed, designated as C-CW without PM addition, K-CW with kaolin addition, M-CW with mixed addition of kaolin and PS, and PS-CW with PS addition. The CWs with or without PM addition showed no significant difference in terms of NH4+-N removal efficiency (p > 0.05), while the removal efficiency of NO3−-N and TN was significantly improved in PS-CW but, in contrast, was considerably inhibited in K-CW and M-CW (p < 0.05). The CWs with PM addition reduced the porosity of the substrates. There was no significant difference in the total solid quality among the CWs with PM addition (p < 0.05), but PS-CW had the highest volatile solid content. The addition of 100 mg/L PS significantly increased the activities of nitrite reductase (NIR) and nitrate reductase (NAR) with a much higher relative abundance of denitrifying bacteria, but it inhibited ammonia monooxygenase (AMO) and nitrite oxidoreductase (NXR) activities (p < 0.05). The activities of the four enzymes were improved to different degrees in K-CW and M-CW, in which the abundance of nitrifying bacteria was higher than that in C-CW. In conclusion, it was noteworthy that the effect of the PMs on the NO3−-N and TN removal performance were qualitatively different (i.e., enhanced vs. inhibited) with different types of PMs. This interesting and important new finding could provide valuable information for a better understanding and evaluation of the role of PMs in the nitrogen removal process during CW operation.