The invention discloses a gas extraction radius determining method for a strike long drilled hole. The gas extraction radius determining method for the strike long drilled hole comprises the following steps that (1) at least one strike long drilled hole is formed in a constructed coal bed; (2) an observation point G[1] is arranged on one side of a lane; (3) an observation hole G[1]K[1] is formed at the position of the observation point G[1], and a negative pressure meter G[1]K[1]B[1] is installed; (4) whether the pressure value on the negative pressure meter G[1]K[1]B[1] drops to a required value P[0] is judged through observation till the pressure value on a negative pressure meter G[1]K[n]B[n] drops below the required value P[0]; (5) the step (3) is executed, an observation hole G[1]K[n+1] is formed at the position of the observation point G[1], and whether the pressure value on a negative pressure meter G[1]K[n+1]B[n+1] drops below the required value P[0] is judged through observation till the pressure value on a negative pressure meter G[1]K[n+m]B[n+m] does not drop below the required value P[0]; (6) it is determined that the gas extraction radius R of the strike long drilled hole formed in the position of the observation point G[1] ranges from G[1]K[n]H[n] to G[1]K[n+m]H[n+m]. According to the gas extraction radius determining method for the strike long drilled hole, the defects of a traditional gas extraction radius determining method for a drilled hole are overcome, and a novel scientific, efficient, accurate and low-cost method is provided for determining the gas extraction radius of a strike long drilled hole.