1. A depth image acquiring device, comprising: at least one projecting device, projecting a projection pattern to a measured object; a first and a second image sensing devices, receiving focal length adjusting signals to adjust focal lengths thereof, and respectively sensing the projection pattern projected to the measured object to generate a first real image and a second real image; a mechanism device, coupled to the first and the second image sensing devices, and receiving lens adjusting signals to adjust locations and/or convergence angles of the first and the second image sensing devices; a processing unit, coupled to the at least one projecting device, the first and the second image sensing devices and the mechanism device, providing the focal length adjusting signals and the lens adjusting signals to adjust the focal lengths, locations and/or convergence angles of the first and the second image sensing devices, and performing a depth operation on the first and the second real images to generate depth information, wherein the processing unit calibrates and obtains a three-dimensional (3D) measuring parameter set of the first and the second image sensing devices at a model focal length according to a plurality of image setting parameter sets corresponding to the model focal length and a plurality of predetermined node distances corresponding to each of the image sensing devices, and accordingly performs a depth operation.