Why the edge of the Pacific Ocean is often called the ring of fire?

Because it is rimmed with volcanoes.

"Volcanoes are not randomly distributed over the Earth's surface. Most are concentrated on the edges of continents, along island chains, or beneath the sea forming long mountain ranges. More than half of the world's active volcanoes above sea level encircle the Pacific Ocean to form the circum-Pacific "Ring of Fire." In the past 25 years, scientists have developed a theory -- called plate tectonics -- that explains the locations of volcanoes and their relationship to other large-scale geologic features."

The ring of fire is a belt that contains most of the active volcanoes of the world. The belt stretches all around the pacifc's edge.

i don't know u tell me!!!

On the shores of the Pacific Ocean (not just the U.S. coast, but Japan and many Polynesian islands) are a series of active and large volcanoes. If you were to map the major or active volcanoes, you would notice they form a kind of "ring," hence "ring of fire."

The "edge" you're referring to is on the edge of a tectonic plate. There are active, erupting volcanoes along the edge of the plate.