With the increase of adjustable loads connected to power systems, there is a growing risk of intentionally exploiting load adjustability to cause power balance deviations. This poses an obvious threat to power system frequency, which has been noticed by many system operators. However, the current dispatch method cannot consider the attack of load on frequency, let alone providing a preventive dispatch decision that reduces the frequency deviation. To address this challenge, a preventive dispatch method that mitigates the impact of adjustable load on power system frequency is presented for the first time. The relationship between the time-varying power mismatch caused by the attack of adjustable load and frequency deviation is established. This is achieved by theoretically deriving the transfer function between system frequency and power mismatch in time domain and discretizing the function into several segments according to the division of time horizons. A formal analysis of the error caused by the transformation is performed, which provides quantitative guidance on the accuracy of the frequency modeling. Based on this, the worst-case scenario of frequency deviation with adjustable load is determined through solving an optimization model. Then, a novel preventive dispatch method that guarantees the system frequency security under the worst-case scenario is presented. Particularly, the generator ramping behavior after receiving the AGC adjustment command is modeled by a group of linear constraints to distinguish the load tracking abilities of generators. Case studies based on the IEEE30-bus system and a 661-bus utility system show that the proposed preventive dispatch method can achieve a 5.8%-19.42% improvement of the maximum absolute frequency deviation.