The narrative structure of "Porphyria's Lover" is linear: it tells a story in sequence from start to finish. It starts by setting the scene with a description of the stormy weather, then moves to Porphyria's arrival, her making herself at home, her sexual advances, and the narrator's strangling of her, ending with the two of them sitting together, him alive and her dead.
The poem is structured as a dramatic monologue, meaning it is told in the first person. As the term monologue indicates, we hear only the point of view of the narrator: the dead Porphyria never gets to speak or respond to the narrator's explanation of events. Therefore, the narrative is highly subjective: we don't know if what the author says is accurate, despite the detail he provides.
A dramatic monologue is a study of the character of a speaker who inadvertently reveals himself to his audience. In this case, despite the matter-of-fact, ordinary, detached recounting of events by this speaker, what he is has done is anything but ordinary and reveals him to be mentally disturbed. The narrative also ends in a sense in the middle of the story: the speaker says
And all night long we have not stirred,
And yet God has not said a word!
We as readers know the other shoe is ready to drop, as someone is going to find him with the strangled corpse of Porphyria: the narrator might imagine all is well, but God—or at least the law—will inevitably be speaking.