This study adopts the acoustic emission (AE) method to measure partial discharges (PD) in a cast-resin dry-type transformer and proposes a waveguide atop the high-voltage winding of the transformer to improve the propagation of the PD acoustic signals occurring in said winding to acoustic emission sensors used. A waveguide is proposed by us to mend the signals’quick attenuation amid the winding’s coils and to eliminate the discharge, encountered by the present methods, from the winding to the sensor. We also study how to analyze the AE signals and how to locate a PD’s source, down to the coil level. Finally, a waveguide is actually installed on a real cast-resin dry-type transformer; the measurements and the locating of PDs’ AE signals for said transformer are carried out. This proposed waveguide forms a new acoustic-wave propagation route to resolve the acoustic-wave propagation difficulty and, meanwhile, eliminate the high-voltage winding’s discharge danger to the sensor. This study demonstrates a feasible new simple, safe and economical solution for diagnosing and locating PDs in the transformer.