Porous carbon nanosheets and corresponding heteroatom doped porous carbon nanosheets have shown great potential as active materials for energy conversion and storage in recent years. However, it remains great challenge to prepare such kind of new two-dimensional (2D) polymer nanosheets without using any templates. In this work, thiadiazole-containing expanded heteroazaporphyrinoid was designed as the building blocks for preparation of free-standing N/S-containing polymer nanosheets (PN) without using any templates. Most importantly, such PN can coordinate with transition metal ions to prepare Fe, N, and S containing PN-Fe. By using these PN-Fe as precursors, Fe/N/S co-doped porous carbon nanosheets (PCN-FeNS) can be facilely prepared by direct pyrolysis under inert condition. The N and S contents of PCN-FeNS can reach up to 6.4 at.% and 0.8 at.%, respectively. For proof-of-concept, PCN-FeNS were further used as electrochemical catalysts for oxygen reduction reaction (ORR) in both alkaline and neutral media. Benefiting from the high surface area and rich-doping character, PCN-FeNS exhibited relatively high half-wave potential of down to 0.71 V, via a four-electron transfer mechanism (n = 3.87 at 0.65 V), as well as high diffusion limiting current density (JL = 5.02 mA cm-2), which are comparable to commercial precious metal based electrocatalysts. This study not only offers a new method to prepare conjugated polymer nanosheets, but also provides a new strategy to fabricate Fe/N/S co-doped porous carbon nanosheets for versatile energy-related applications.