Abstract: Fractional frequency transmission technology has gradually become one of the mainstream solutions for realizing carbon neutrality and long-distance transmission of offshore wind power. However, the research on electrical equipment in the context of high-voltage FFTS is rarely available, and the effects of frequency change on the characteristics of cable insulation materials need to be studied in depth. To this end, a pulsed electroacoustic test system capable of rapid detection was used to investigate the space charge characteristics of low-density polyethylene materials at crossover voltages with different field strengths and temperatures as well as the effect of frequency change on the charge build-up behavior. The results show that the charge accumulation is obvious when the field strength and temperature conditions are more severe; while the charge accumulation becomes more remarkable with the decrease of frequency, and the charge growth rate increases significantly when the frequency decreases below 20 Hz. Based on this, a bipolar carrier model applied under alternating electric field was developed, and the experimental observations were rationalized by simulation calculations to reveal the mechanism of the influence of electric field, temperature and frequency on the space charge behavior. The research methodology and results can provide theoretical supports for the frequency choice and optimal design of cable insulation in frequency-divided transmission systems.