A harness for backpack having an adjustable longitudinal length is disclosed. The harness has a top section and a bottom section slidably engaging one to each other; at least one holding system extending from the top section to the bottom section of the harness for holding the harness on a user's back; and an adjusting element connecting the top and bottom sections and configured for maintaining the longitudinal length of the harness once the longitudinal length is adjusted to a given length adapted to the user's back. A kit for assembling a packsack and methods for adjusting the harness are also disclosed. The harness according to the present invention is easily adjustable and particularly adapted for kids and school packsacks. When the kid is growing in size, the kid can easily adjust the longitudinal length without having to disassemble the harness from the pouch.