The multipath effect can severely reduce the positioning accuracy of GNSS receivers. Thus, it is very important to estimate and mitigate the multipath delay error in tracking algorithms, especially in harsh environments. The code phase error is highly related to the positioning performance. Especially, the absolute position results are dependent on the absolute term of the code phase. However, the absolute code phase is seldom discussed and processed in state-of-the-art methods. In this paper, the absolute position is proposed to form a code phase discriminator to estimate the missing absolute code phase. In this discriminator, the standard vector tracking delay/frequency lock loop (VDFLL) is also used for compensating the change of the code phase error. Besides, the multipath error is estimated with the between-satellite single difference algorithm to further improve the proposed absolute-position-aided (APA) VDFLL code phase discriminator. The real-world static field test demonstrates the proposed algorithm's high performance in positioning and multipath mitigation.