A method is proposed to measure electrical contact Passive Intermodulation(PIM) based on a slot waveguide, which is separable and can simultaneously measure contact resistance and PIM. On this basis, the statistical behavior of metal contact PIM is investigated. Firstly, it is determined that the existence of oxide film on the surface of Device Under Test(DUT) with aluminum and silver coated aluminum, is the main reason for the producing of PIM, by characterizing the surface topography and analyzing the components. Secondly, the contact resistance(Rc) and its statistical regularities are obtained by repeating loading-unloading at some certain pressure with four-terminal sensing method. Finally, the relationship among contact resistance, PIM and contact pressure of aluminum alloy and silver coated aluminum alloy are measured by special designed slot waveguide measurement system. The experimental results show that, the contact resistance of the contact joint has the statistical characteristic under the certain pressure and its PIM value is also fluctuant. Both the contact resistance and PIM value decrease as the contact pressure increases and their fluctuant ranges decrease.