Introducing the long chain branching or cross-linked structure in polypropylene (PP) is highly desired to meet the requirement of high melt strength, which is a key for the application of PP in thermoforming, blow molding and foaming where elongation flows dominate. In this work, the power ultrasound was introduced into the reactive extrusion at the exit die of an extruder for preparing the long chain branched or cross-liked PP. The main goal was to maintain the highly branched and cross-linked structure during reactive extrusion process in superior process-ability case of low peroxide concentration. The results of the dynamic rheological property, melt flow index, dynamic mechanical spectrum and extensional viscosity measurement showed that the cross-linked extent and melt strength of PP increased obviously with the employment of a ultrasound of 300 W while the content of functional monomers and peroxide was constant. FTIR analysis, DSC characterization, and GPC measurement proved that the increase of cross-linked extent and melt strength was attributed that the ultrasonic waves can induce the chain scission and recombination reaction when the PP melts flowed through the exit die assembled with a ultrasonic probe.