This work combined the grafting maleic anhydride(MAH) onto polypropylene (PP) and the coupling reaction between diamine and MAH grafted PP (PP-g-MAH) into the one-pot reactive extrusion. Detailed structure analysis and rheological characterization of the resultant long chain branched polypropylenes (LCBPPs) were conducted subsequently. The results indicated that the concentration of the relative reagents plays a key role in control of the long chain branched structure. Moreover, the foaming behaviors of the resultant modified polypropylene assisted with supercritical carbon dioxide (scCO2) were investigated and the novel performance could be obtained. This study not only revealed the effect of the variation of initial reagents concentration on the branching modification, but also showed a simple one-pot strategy concerning the process optimization for the preparation of LCBPPs.