Cr4+:YAG is an attractive gain medium due to its broad 3-dB emission spectrum ranging from 1253 to 1530 nm, which cover the low-loss window of the silica fiber. Such a broadband haracteristic offers a potential to develop a broadband amplified spontaneous emission (ASE) light source, optical amplifier, and tunable laser. Growing the Cr4+:YAG bulk crystal into fiber form is necessary for generating larger gain by the better optical confinement of the waveguide structure. For the application of laser, it is superior to bulk crystals for reducing lasing threshold and obtain the high slope efficiency due to the beter optical confinement and heat dissipation. A record-low threshold of 0.75 mW was achieved at room temperature. It is more than 500 times lower than any previously reported Cr4+:YAG lasers, and a slope efficiency of 6.9% was obtained. The ultralow-threshold lasing is made possible by the low propagation loss of 0.08 dB/cm and the high pump intensity of the core. Such a low-threshold operation makes the double-clad crystal fiber laser be compatible to present optical communication systems. Furthermore, we demonstrate that the proposed optimization algorithm can provide an efficient and direct approach to analyzing both fiber structure and bulk medium in terms of practical application requirements. The simulation results reveal that Cr4+:YAG DCF lasers offer advantages over bulk lasers in terms of low-threshold lasing and high efficiency, which are essential for their use in optical communication systems.