The utility model provides an adjustable plant height measuring ruler comprising a ruler slot and a slide range pole plugged in the ruler slot in a slide manner; the ruler slot is provided with a first scale bar and a second scale bar having same length and direction and value changing sequence; the bottom of the slide range pole is provided with a pointer pointing to the second scale bar. The adjustable plant height measuring ruler can measure a plant with the height double the ruler slot length; the slide range pole can be contracted in idle state, thus reducing length, and the ruler is portable and convenient in usage; the pointer is convenient for accurate reading; the ruler is reasonable in design, practical and labor saving, accurate in measurement, and convenient in usage, and very suitable for promotion application in measuring a plant height, especially a tall planet test.