The invention relates to a precamber measuring method in steel box girder construction. The simple precamber measuring method in the steel box girder construction is characterized by comprising the following steps of: (1) installing a precamber measuring device at the intersection point of the road centre line of a steel box girder and the centre line of a first pier; (2) fixing the mounting height of an infrared laser pen of the precamber measuring device; (3) installing a dividing rule on the steel box girder at the centre line of a second pier being adjacent to the first pier, starting theinfrared laser pen of the precamber measuring device, leading infrared laser generated by the infrared laser pen to be indicated to the dividing rule and reading out the indicated numerical value h1 of the dividing rule; and (4) moving the dividing rule to the midspan or the preprecamber value position appointed in design paper, and reading out the numerical value h2 of the dividing rule, indicated by the infrared laser, wherein the difference value of h2 and h1 is the preprecamber value of the steel box girder. The method is simple and easy to realize.