Method for the ink level detection of thermal bubble ink-jet printer was studied in the paper. Some past and current methods of ink level detection for thermal bubble inkjet printers were reviewed first in the paper. Disadvantages of them could be found. Therefore, the fundamental physical principles (fluid dynamics) have been discussed to understand what physical properties would be changed as the ink being consumed in the ink reservoir. It's found that the natural and close relationship between the pressure and ink level could be applied to predict continuously the ink volume contained in the reservoir. Following the relationship, one new method, including three different schemes herein, have be presented explicitly for doing such an implementation by a pressure transducer which is available at the present day. A simple procedure was also described to show how the data process and transformation should be manipulated. The new presented method here will own many advantages, including no waste of ink, early warning of ink out, and so on.