A downhole adjustable drilling inclination tool including an outer housing, an inner housing, a compression spring, a piston assembly, and a tilt housing. The piston assembly is fluidly controlled to move axially along the outer diameter of a bottom end portion of the inner housing so that a rotatable control ring moves about a guide pin to hold a neutral, straight, or bent position of the piston assembly corresponding to an amount of compression of the compression spring. The tilt housing partially disposed within the outer housing includes a bolt plate pin channel configured to receive a bolt plate pin of the piston assembly that travels to tilt the tilt housing by a tilting mechanism that connects the tilt housing to the outer housing and position the tilt housing in a neutral, straight, or bent position corresponding to the neutral, straight, or bent position of the piston assembly.