Abstract: The high range resolution can resolve the target into a number of scattering centers. And under high-range resolution radar mode, the target can be spatially distributed across some contiguous range cells to form a range-spread target. The range-spread target's correlation of different range cells should not be ignorable. The effect of the spatial correlation of range-spread targets on the adaptive matched filter (AMF) detector is studied. The resolutions show that, if both the covariance matrix structure of clutter Rc and the mode matrix of range-spread target H are the diagonal matrix, the tasks of range-spread target detection can be implemented at each cell under test respectively and independently. And the final decision statistics is the summation of statistics in every cell which is occupied by the range-spread target. It is shown that, the number of range cells L, which are occupied by the range-spread target, has significant effect on the signal-to-clutter ratio. That is to say, inaccurate estimate for L can lead to the signal-to-clutter ratio loss. It is shown that, the spatial correlation of range-spread target among cells under test can make the range-spread target degenerate into the point target.