The present invention provides a method for detecting horizontal height between plural parallel-arranged plates, which comprises a cross-shaped and hybrid intersecting path according to the plural arrays of heat dissipating plates, and uses at least two vertically-hanging probes in the manner of relatively oblique and relatively parallel probing to contact the edges of two adjacent heat-dissipating plates for detection of the combined height between two adjacent heat-dissipating plates. The present invention further provides a kind of horizontal height inspection apparatus between the plural parallel-arranged plates for realizing the above-mentioned method; therefore, the problem of lacking automation during the assembly process of the LED light box is improved to enhance the production yield.