An axle locking cylinder structure for heavy construction equipment, including a locking cylinder connected to a supply flow path which communicates with a main hydraulic pump, and a return fluid path which communicates with a pressurized reservoir, the locking cylinder being supplied with a hydraulic fluid through the supply flow path; a cylinder rod slidably moved in the locking cylinder by a pressure of the hydraulic fluid supplied to the locking cylinder; and a pilot valve installed between the supply flow path communicating with the main hydraulic pump and a pilot flow path communicating with a cylinder chamber in the locking cylinder; the pilot valve comprising: a first pilot flow path being feedable with hydraulic fluid through supply flow path to provide a pilot pressure; a poppet valve moved up and down by the pilot pressure to selectively open and close the pilot flow path connected to the cylinder chamber; a pressurized chamber formed between the poppet valve and the pilot flow path, the pressurized chamber is in communication with the cylinder chamber through the pilot flow path; a valve spring resiliently supporting the poppet valve downward to close the pilot flow path; an adjusting plug coupled to an upper portion of the locking cylinder for adjusting the resilient force of the valve spring; a second pilot flow path feeding the hydraulic fluid drained from the cylinder chamber to the return fluid path when the pilot flow path is opened; characterized in that the poppet valve having a poppet stepped portion extending from a poppet seat portion towards a front end thereof and having a diameter smaller than that of the pilot flow path and further comprising: a branched flow path formed between the cylinder chamber and the supply flow path and connected in parallel with the poppet valve to supply the hydraulic fluid to the cylinder chamber when the cylinder rod is expanded; and a check valve installed to one side of the branched flow path; wherein the check valve is fed with a certain pilot pressure through a third pilot flow path which communicates with the pressurized chamber, and has a valve plunger for selectively opening and closing the branched flow path by using the pilot pressure. a first pilot flow path being feedable with hydraulic fluid through supply flow path to provide a pilot pressure; a poppet valve moved up and down by the pilot pressure to selectively open and close the pilot flow path connected to the cylinder chamber; a pressurized chamber formed between the poppet valve and the pilot flow path, the pressurized chamber is in communication with the cylinder chamber through the pilot flow path; a valve spring resiliently supporting the poppet valve downward to close the pilot flow path; an adjusting plug coupled to an upper portion of the locking cylinder for adjusting the resilient force of the valve spring; a second pilot flow path feeding the hydraulic fluid drained from the cylinder chamber to the return fluid path when the pilot flow path is opened; characterized in that the poppet valve having a poppet stepped portion extending from a poppet seat portion towards a front end thereof and having a diameter smaller than that of the pilot flow path and further comprising: a branched flow path formed between the cylinder chamber and the supply flow path and connected in parallel with the poppet valve to supply the hydraulic fluid to the cylinder chamber when the cylinder rod is expanded; and a check valve installed to one side of the branched flow path; a branched flow path formed between the cylinder chamber and the supply flow path and connected in parallel with the poppet valve to supply the hydraulic fluid to the cylinder chamber when the cylinder rod is expanded; and a check valve installed to one side of the branched flow path;