This paper presents a mobile 3D coordinate measuring system for large scale metrology. This system is composed of a network of rotating laser automatic theodolites (N-RLATs) and a portable touch probe. In the N-RLAT system, each RLAT consists of two laser fans which rotate about its own Z axis at a constant speed and scan the whole metrology space. The optical sensors mounted on the portable touch probe receive the sweeping laser fans and generate the corresponding pulse signals, which establish a relationship between rotating angle of laser fan and time, and then the space angle measurement is converted into the corresponding peak time precision measurement of pulse signal. The rotating laser fans are modeled mathematically as a time varying parametrical vector in its local framework. A two steps on-site calibration method for solving the parameters of each RLAT and coordinate transformation among the N-RLATs. The portable probe is composed of optical sensors array with specified geometrical features and a touch point, on which the coordinates of optical sensors is determined by the N-RLATs and the touch point is estimated by solving a non-linear system. A prototype mobile 3D coordinate measuring system is developed and experiment results show its validity.