In 5-dimensional space-time, the Planck quantum gravity theory can be constructed based on standard quantum field theory methods. The 4-dimensional space-time observed by humans is a mapping of the 5-dimensional space-time. The mapping from 5-dimensional space-time to 4-dimensional space-time satisfies the path integration rule. By using the path integration method, the space-time mapping can be deduced and calculated, so as to transform the Planck quantum gravity theory of 5-dimensional space-time into Planck gravity theory in 4-dimensional space-time, and then calculate and explain the gravitational phenomena in 4-dimensional space-time, which is consistent with the experimental results in 4-dimensional space-time. Physically, the mapping from high-dimensional space-time to low-dimensional space-time requires the use of path integration rules, which is the spacetime mapping principle. The gravitational mass in 4-dimensional space-time comes from the space-time mapping, and the gravitational mass is just the inertial mass, and there is no independent gravitational mass. The physical meaning of mass in gravity is to denote a new type of probability density. In the 4-dimensional space-time, Planck quantum gravity theory has different calculation results from other gravitational theories in five aspects, and whether Planck quantum gravity theory is consistent with the real universe can be tested experimentally.