[Objective] The study aims to investigate the physiological mechanism of biochar with nitrogen application on mitigating the germination of paulownia seeds under Cd stress, and to provide a reference for the application of biochar and nitrogen for remediation of Cd-contaminated soil. [Methods] Taking the seeds of "Paulownia 1201" as the test material, we investigated the effects of applying different concentrations of Paulownia biochar treatment solution with nitrogen on the germination characteristics, antioxidant levels, osmotic substances and nutrient levels of Paulownia seeds under Cd stress. [Results] (1) The single application of biochar and nitrogen or the combination of the two could promote seed germination, and the seed germination potential, germination rate, germination index, and vitality index were all increased. The length, root length, fresh weight, SS, SP, and free amino acid content of young shoots increased with the increase of treatment solution concentration and reached the maximum value under 5 g/L biochar treatment, while starch content decreased. (2) When biochar or nitrogen was applied alone, GR, Pro, and MDA contents were significantly decreased, while H2O2 contents were not significantly decreased, but Pro and H2O2 contents were all minimized under 5 g/L biochar treatment. In addition, APX activity showed a tendency of increasing and then decreasing with the increase of germination time. (3) When the two were combined, APX and GR activities reached the maximum value, MDA and H2O2 contents were significantly reduced, and Pro content increased but not significantly. The overall improvement effect was biochar with nitrogen > biochar > nitrogen. [Conclusion] The application of biochar and nitrogen can enhance the antioxidant and osmoregulation ability of paulownia seeds, promote the accumulation of nutrients, and significantly promote seed germination, especially the best effect of biochar with nitrogen treatment.