A slotted line system is constructed with simple devices. The wavelengthsλF of different positions at the axis of spot-focusing lens antenna are measured based on the method of standing wave. By comparing with wavelength λ0 of free space, the relation that λF changes with the axial length is found, and qualitatively analyzed. Through the actual measurement on two kinds of spot-focusing lens antennas，it can be confirmed that the wavelength λF near the focal spot has a elongation of 0 to 5 percent in comparison to λ0 in free space, and this elongation will reduce with the increase of the distance to the focus. Finally, precision and error analysis of the test system is given.