Pop music jamming on the keyboard requires massive music knowledge. Musician needs to understand and memorize the behavior of each chord in different keys. However, most simple pop music follows a common chord progression pattern. This pattern applies to most simple pop music on all the 12 keys. We designed an app that can reduce the difficulty of music jamming on the keyboard by using this pattern. The app displays the current chord in the Roman numeral and suggests the expected next chord in an easy to understand way on a smartphone. This work investigates into the human computer interaction perspective of music performance. We use a smartphone app as a bridge, which assists musician to react faster in music jamming by transforming the complex music knowledge into a simple, unified and easy to understand format. Experiment result shows that this app can help the non-keyboardist musician to learn pop music jamming. It also shows that the app is useful to assist keyboardist in making key transpose and playing music in the key with many sharps and flats. We will use the same interface design to guide user on playing other chord progressions such as the jazz chord progression.