The present invention provides a pressure control apparatus for an ink-jet pen. The apparatus will adjust the back pressure of ink in the reservoir of the ink-jet pen in a suitable range when the ink is used gradually. The apparatus includes a pressure adjusting element and a hole furnished on the bottom wall of a reservoir. Under normal condition, the pressure adjusting element contacts tightly to the hole and prevents air from ingression. When the back pressure increases gradually to an extent, the ambient air will overcome the pressure of a spring which provides force to the pressure adjusting element, thus move the pressure adjusting element to leave a clearance to the hole and let some air bubbles ingressing into the reservoir till the back pressure decreases to a certain value. After the back pressure returns to the certain value, the pressure adjusting element further contacts the hole and prevents air from ingression, thus maintains a suitable back pressure of the ink in the reservoir.