Abstract: Photogrammetry is often processed in Euclidean space. Some investigation on basic photogrammetric problems in projective space is done. The authors are built the collinearity condition equation, the direct equation between object points and image points and the coplanarity condition equation between homologous points are built. Compared with the forms in Euclidean space, they are simpler in description. Moreover, they lower the requirement to the measure system of image points. Image points can be firstly measured in an arbitrary coordinate system on image plane. The value of focus length and the coordinates of principle point lose their concrete meaning, which means the interior orientation elements can be unknown or ignored in calculation. At the same time, the exterior orientation elements are also not required in calculation. These equations are the fundamentals of photogrammetry, Their speciality brings great convenience in the other processing of photogrammetry, and they resolved the problems met by traditional photogrammetry in real-time processing.