IM-5 zeolite is synthesized in solutions of sodium hydroxide, sodium aluminate, and silica gel, along with N-methyl pyrrolidine and 1,5-dibromopentane by one-pot hydrothermal method. The method described in this paper is more convenient and environmentally benign than the conventional one, in which 1,5-bis (methylpyrrolidinium) pentane bromide is used as structure directing agent, because it eliminates the complex and toxic synthesis process of the agent. The influences of synthesis variables including OH−/SiO2 ratio, H2O/SiO2 ratio, temperature, seed content, and crystallization time on the synthesis of IM-5 zeolite are studied with X-ray powder diffraction, field emission scanning electron microscopy and nitrogen adsorption characterization techniques. The experiments show that analcime and mordenite are the two impurity phases that compete with IM-5 zeolite in synthesis process and the alkalinity of solution, temperature, and the crystallization time are three key factors that affect the crystal phase of products. A tentative explanation for the mechanism of the crystal phase dependence on the synthesis variables is given. By carefully choosing synthesis conditions along with using seed, the crystallization time of synthesizing IM-5 zeolite has been greatly reduced.