Abstract: The blister rust of Mongolian pine is an important disease. The pathogenic fungus is identified as Cronartium flaccidum (Alb. Et Schw.) Winter, its telia host under natural conditions in Heilongjiang Province is Paeonia species. The fungus also infects Melaplexis japonica under inoculation. The spreading period in woods for its aeciospore is 56 days and the spore can be caught at the distance of 30m, the vertical is about 5m high with abundant collection of spores. Aeciospore is binucleate, only one germ tube has priority to grow. On the 2% agar water membrane with temperature 20℃ under saturated moisture, and pH about 7 the germination rate reached 80%. The germination rate of uredium can be raised about 15% if exposuring in the sunshine before germination. The germinating rate of urediospore, on 2% agar water membrane under 25℃, reach 78.4% for 48 hours. The overwintered urediospore on diseased leaves of Paeonia species still can germinate (2.96%) and infect. The teliospore will mature in three days after its formation and then germinate under the temperature 16—21℃ and 3 hours wet condition, producing basidiospores. Hyphae beneath the canker bark of Mongolian pine are perennial, septate, monocaryotic, and grow between host cells, entering cells to form haustoria. Some weed killers may eradicate Paeonia leaves. Using 10%, 20% NaCl solution or 5—10 grams, 10—20 grams of NaCl crystal to spread around the roots can kill Paeonia roots. Smearing chemical medicine on the surface of diseased stems, such as N-dodecyl-12.6-dimethyl morpholine (1000 times), the solution of creosote and machine oil mixed with corn germ oil (1:3 or 1:5), will kill aeciospore although there are chemical harm for twigs and young branches. Pruning including cutting off diseased shoots is an ideal measure to promote the growth of forest trees and the disease prevention.