Two goals either side of half-time from Cristian Gomez, Joan Verdu, Alvaro and Kalu Uche handed the hosts a thumping victory over the Mestalla outfit at Cornella-El Prat
Cristian Gomez and Joan Verdu struck in the first half before Alvaro and Kalu Uche capitalised on some slack defending in the second 45 to hand the visitors a heavy defeat just days before their Europa League semi-final first leg against Atletico Madrid.
Buoyed by their midweek demolition of Rayo Vallecano, it was visitors who made the brighter start, with Tino Costa firing an early warning shot straight at Cristian Alvarez after some neat build-up play.
But the visitors' vibrant start soon petered out as the game became bogged down with numerous stoppages, with Espanyol gradually beginning to assert themselves in the middle of the park.
And the hosts' early pressure eventually paid off as they struck two goals in quick succession to take total control of the match.
Firstly, Sergio Garcia skipped past a challenge from Mehmet Topal down the right before squaring the ball for Gomez to finish past Guaita for his first goal of the season.
The second came just four minutes later through a sumptuous long-range effort from Verdu, who was afforded the space to run towards the edge of the Valencia box before unleashing an unstoppable drive in to the right-hand corner of Vicente Guaita's net.
A shell-shocked Valencia took some time to recover from the double blow and nearly conceded a third, but Garcia was unable to get on the end of Vladimir Weiss' driven cross across the face of goal.
Although they enjoyed superior possession, the visitors' final ball was often found wanting as they struggled to fashion clear openings in front of goal.
Emery responded to his team's limp showing with a double substitution at half-time, with Aritz Aduriz and Sofiane Feghouli coming on for Dani Parejo and the anonymous Pablo Hernandez.
The changes instilled some much-needed urgency to the visitors' play, and Alvarez had to be alert to clear a through ball with Jeremy Mathieu lurking.
But Los Che had the wind blown out of their sails in the 58th minute, as Espanyol stretched their lead further courtesy of an awful error from Guaita.
Under no pressure, the shot-stopper played a poor pass out to Topal, who was caught in possession by Garcia, who then set up substitute Alvaro for a simple tap-in from close range.
Los Che could not muster much of a fight-back and it was the hosts who looked the more likely to score, with Verdu having a free kick clawed away by Guaita before Uche completed the rout with a finish from a tight angle after he was played in by the excellent Garcia.
Valencia now travel to the capital on Thursday for the first leg of their Europa League semi-final against Atletico, while Espanyol's next assignment also sees them visiting the Vicente Calderon in La Liga next weekend.