Heat oven to 350°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray.
Place pretzels in resealable food-storage plastic bag; smash with rolling pin or flat side of meat mallet until crushed. In medium bowl, mix crushed pretzels, brown sugar and pumpkin pie spice. Stir in butter. Press mixture onto bottom of baking dish. Bake 9 to 11 minutes or until set. Cool on cooling rack.
Meanwhile, in large heatproof bowl, pour boiling water on gelatin; stir until gelatin is dissolved. Add enough cold water to reserved juice to equal 2 cups; stir into gelatin. Refrigerate until partially set, about 1 hour 45 minutes.
In medium bowl, beat cream cheese and granulated sugar with electric mixer on medium speed until light and fluffy. Fold in whipped topping. Spread mixture evenly over pretzel crust.
Finely chop mandarin orange segments; gently stir into partially set gelatin. Carefully pour over cream cheese layer. Refrigerate 8 hours or overnight. Cut into squares.