To address the problems of traditional phase-locked loops （PLLs），including slow dynamic response，poor detection accuracy，and insufficient suppression capability under complex operating conditions such as grid voltage unbalance and sudden frequency changes in flexible DC transmission systems，an adaptive PLL control strategy based on improved second-order linear active disturbance rejection control （LADRC） is proposed.This strategy employs a cross-decoupled dual complex coefficient filter with a frequency feedback loop as a pre-filtering stage to extract and separate the positive- and negative-sequence components of the voltage.In addition，a first-order feedforward derivative stage is introduced at the input reference signal of the second-order LADRC，and an improved second-order LADRC controller is designed as the core module to accurately detect and lock the frequency and phase of the voltage positive-sequence component.A flexible DC grid simulation model is established on the PSCAD/EMTDC platform for simulation and verification.The results demonstrate that the proposed adaptive PLL control strategy exhibits fast dynamic response，strong anti-interference capability，and high tracking accuracy for grid frequency and phase，and can effectively eliminate the influence of voltage unbalance and harmonics on the PLL.