Talking head generation based on neural radiance fields (NeRF) has gained prominence, primarily owing to its implicit 3D representation capability within neural networks. However, most NeRF-based methods often intertwine audio-to-video conversion in a joint training process, resulting in challenges such as inadequate lip synchronization, limited learning efficiency, large memory requirement, and lack of editability. In response to these issues, this paper introduces a fully decoupled NeRF-based method for generating talking heads. This method separates audio-to-video conversion into two stages through the use of facial landmarks. Notably, the Transformer network is used to effectively establish the cross-modal connection between audio and landmarks and to generate landmarks conforming to the distribution of training data. We also explore formant features of the audio as additional conditions to guide landmark generation. Then, these landmarks are combined with Gaussian relative position coding to refine the sampling points on the rays, thereby constructing a dynamic NeRF conditioned on these landmarks and audio features for rendering the generated head. This decoupled setup enhances both the fidelity and flexibility of mapping audio to video with two independent small-scale networks. Additionally, it supports the generation of the torso from the head-only image with improved StyleUnet, further enhancing the realism of the generated talking head. Our experimental results demonstrate that our method excels in producing lifelike talking heads, and that the lightweight neural network models also exhibit superior speed and learning efficiency with lower memory requirements.