In this work, the effects of the pairing correlation on the properties of neutron drops $N$=6-50 trapped in a harmonic oscillator potential with $\hbar\omega=10$ MeV are investigated by comparing the results given by the Skyrme Hartree-Fock and Hartree-Fock-Bogoliubov theories with the density-dependent delta interaction (DDDI) pairing force. The results showed that the pairing correlation slightly made the neutron drops more bound, and increased the central neutron density, the spin-orbit and pseudo spin-orbit splittings. Thus, the pairing correlation must be accounted for to improve the Skyrme functional compared with the ab initio calculations. Furthermore, although the single-particle energy gaps with or without pairing were similar, the shell closures varied due to pair scattering. Here, the shell closures in neutron drops using the SkM* parameter set and DDDI pairing force were found at $N$=8, 16, and 32.