A charging base includes a charging cradle, a pad plate, and a recess positioning portion. A mobile device includes a bottom base, a contact portion disposed on the bottom base, and a drive wheel drives the mobile device to move. The charging cradle includes a platform and a charging contact portion, and a height of the charging contact portion is greater than a height of the bottom base, when the drive wheel moves onto the pad plate, the height of the bottom base is greater than the height of the charging contact portion, when the drive wheel reaches the recess positioning portion while moving from the pad plate toward the charging cradle, the height of the bottom base is less than or equal to the height of the charging contact portion, so that the bottom base covers the platform and the contact portion is in contact with the charging contact portion.