Titanium tailing slag, a chlorinated waste resulting from titanium extraction processes from high-titanium blast furnace slag, has historically lacked viable technologies for large-scale utilization, posing potential environmental risks. This study leveraged titanium tailing slag as the main ingredient in the development of thermal insulation materials, specifically calcium silicate boards, and evaluated the influences of various parameters such as calcium to silicate ratio, diatomite content, water-to-binder ratio, and mix proportion of raw to dechlorinated slag on the material’s performance metrics, phase composition, and microscopic morphology. Findings indicate that the flexural strength of calcium silicate boards exhibits a pattern of initial increase followed by a decrease as the proportion of dechlorinated slag is raised. Excess diatomite, high calcium to silicon ratio, and elevated water-to-binder ratio adversely affect the enhancement of sample attributes. Optimal material properties are observed when the composition included 46% (mass fraction) titanium tailing slag (with an equal ratio of raw to dechlorinated slag), 15% (mass fraction) silicate cement, 17% (mass fraction) diatomite, a calcium to silicon ratio of 0.84, and a water-to-binder ratio of 0.75. Under these conditions, the calcium silicate board demonstrates superior characteristics: a bulk density of 1.25 g/cm3, a flexural strength of 10.4 MPa, a thermal conductivity of 0.297 W/(m·K), and a sample-contained chloride ion mass fraction of 0.36%, achieving a 56% chloride fixation ratio.