The results of HgCdTe long-wavelength infrared n+-on-p planar photodiode arrays passivated by molecular beam epitaxy (MBE) in-situ grown CdTe film were presented in this paper. By mercury-vacancy p-type annealing, ion-implantation window exposure, ZnS ion-implantation barrier layer deposition, B+-implantation, ion-implantation barrier layer removal, ZnS dielectric film deposition, metallization and indium-bump arrays fabrication, HgCdTe long-wavelength infrared n+-on-p planar photodiode arrays using in-situ CdTe passivation was achieved from a Hg1-xCdxTe film covered with a layer of MBE in-situ grown CdTe film. Zero bias dynamic resistances of HgCdTe photodiode arrays using in-situ CdTe passivation were improved 1~2 times higher than those of non-in-situ CdTe passivation processed one, and the maximum dynamic resistances near small reverse biases were even increased by a factor of 30~40. Since their current-voltage curves were all measured at 78K, it is obvious that in-situ CdTe passivation was beneficial to suppress dark current of n+-on-p planar photodiode by optimizing the interface between the HgCdTe detector and CdTe passivation layer, and then to enhance the performance of long-wavelength infrared photodiode arrays operating at small reverse biases.