An oscillating range adjusting module with modular design for being mounted into a sprinkler is revealed. The oscillating range adjusting module which includes a first adjusting unit and a second adjusting unit is located between a water-in control module and an oscillation driving module. During assembly, the first adjusting unit is fitted on an inlet tube of a water-in control module. After an O-ring being connected to the first adjusting unit, the second adjusting unit is mounted on the inlet tube and connected to the first adjusting unit by at least one fastener of the second adjusting unit engaged with at least one connection surface of the first adjusting unit. Lastly a water inlet of the oscillation driving module is connected to the second adjusting unit. Thereby, a modular assembly of the oscillating range adjusting module between the oscillation driving module and the water-in control module is achieved.