Focusing on the financing barriers to firm productivity improvement under the influence of external shocks, we empirically analyze the data of A-share listed companies from 2007-2018 to determine the impact of financing constraints on total factor productivity (TFP) in the context of COVID-19 pandemic and the paths of factor use efficiency and R&D innovation efficiency on this impact using ordinary least-squares (OLS) method. We find that financing constraints are an important factor inhibiting the TFP of firms. This inhibitory effect is more serious in small-scale firms, non-state firms, and non-energy firms. Further investigation shows that the inhibitory effect of financing constraints on firms' TFP is more pronounced when firms are located in the Yangtze River Delta city cluster, the Pearl River Delta city cluster, non-port cities, and provincial capitals. The mechanism test finds that improving the efficiency of capital use and labor use can alleviate the suppressive effect of financing constraints on TFP. The alleviating impact is more significant when capital use efficiency is improved. However, increasing the efficiency of R&D innovation further strengthens the inhibitory effect of financing constraints, and this effect is more pronounced under positive external shocks.