Paul Muldoon`s poems often lack the autonomous structure ordinarily expected from literary works by most modernists. Critical interest in his charm is more or less triggered by the postmodern opaqueness of his language. This article is an effort to look into how his language has to be difficult and how it achieves its validity, by examining “At the Sign of Black Horse”, the last and only long poem in his 2003 Pulitzer prize winning book, Moy Sand and Gravel. Muldoon`s poetry gets inevitably difficult so long as he accepts the world as uncertain in itself. His characteristic difficulty can be more clearly accountable when approached from four causes in terms of its contents as well as techniques. First, his poetry comes from multi-cultural experiences. As his realistic sense not only comprehends the personal but also the historical, Muldoon`s experiences, covering the Irish, English, American, and Jewish, compel him to see the world from multi-layered contexts. Second, Muldoon is inclined to draw his subject matters from unfamiliar resources. A vast array of philosophical or historical materials are provided as fragments without argument. Third, Muldoon habitually uses the technique of interpolating or mixing various fragments in an unexpected way. The poem proceeds depending primarily on association. The transition from one fragment to another looks arbitrary and impromptu, but as the poem goes on, some connections, among those fragments near or far, though not from consequential directness, seem to come into being. When the connections completement one another in their similarities, and get strong enough to make up a certain story, they finally work in the way the autobiographical, historical, and philosophical are all comprehensively considered. Nevertheless, these connections do not necessarily lead up to the autonomous structure of the work. Fourth, Muldoon is hard to read because he bears a playful attitude toward poetry and the world. His cold detachment, when dealing with historical violences affecting his own children`s future, shows that he is less focused on political beliefs than on poetic forms or language itself. The more he abstains from political comments, the more difficult his poem becomes to get hold of. “At the Sign of Black Horse” is a three-hundred-and-sixty-line poem where a variety of knowledge from multi-layered experiences, autobiographical or historical, are mingled together. The connections among fragments seem to be whimsical at a glance, but sometimes articulate at another. The poem reaches a contradictory balance when it seeks the precisely controlled language on the one hand, and the whimsical imagination on the other, and thus may well be a product of deliberate whimsy.