An inkjet printing system and method capable of automatically calibrating non-uniform speed of a printhead carriage, which includes a test pattern generator, a printing device, an image extractor, a processor and a printhead carriage control signal generator. The test pattern generator generates at least one test pattern. The printing device prints the test pattern on a recording medium. The image extractor captures an image of the test pattern. The processor reads and analyzes the image of the test pattern to accordingly generate a look-up table. The printhead carriage control signal generator generates an inkjet control signal for the printhead carriage, and adjusts a clock of the inkjet control signal in accordance with the look-up table.