The neutron lifetime is one of the important parameters for particle physics and astrophysics. There is a 4.6$\sigma$ (9.5 s) discrepancy between the results of the two typical methods. To solve this discrepancy, a new neutron lifetime experiment with a different method is in progress at J-PARC, and this experiment published the first result in 2020 of $898 \pm 10 (\mathrm{stat.}) +15/-18 (\mathrm{syst.})\,\mathrm{s}$. We have performed improvements to reduce systematic uncertainties towards 1 s accuracy. The methods to measure the amount of $\mathrm{^3He}$ injected and in the working gas have been upgraded, and a new model for the space charge effect in the Monte Carlo simulation has been implemented.