NOVELTY - Preparation of modified mifepristone intermediate involves: taking magnesium and ethyl bromide as raw materials, reacting at 25-39 degrees C in presence of tetrahydrofuran to obtain ethyl magnesium bromide; inletting acetylene gas in ethyl magnesium bromide, reacting to obtain propynyl magnesium bromide; adding 3-alpha,3-beta-ethylene-dioxide-5(10),9(11)-diene-17-ketone in propynyl magnesium bromide, reacting to obtain 3-alpha,3-beta-ethylene-dioxide-17-beta-hydroxy-(1-propynyl)-5(10),9(11)-diene; and collecting the compound.    USE - For preparation of modified mifepristone intermediate (claimed).    ADVANTAGE - The method provides improved method preparation of modified mifepristone intermediate, as compared to the prior art methods, where the method has improved yield and purity of product, reduces raw material consumption, and reduces environmental pollution. The method improves purity of propynyl magnesium bromide, and mifepristone intermediate 3-alpha,3-beta-ethylene-dioxide-17-beta-hydroxy-(1-propynyl)-5(10),9(11)-diene, lowers production cost, and is environment friendly. The reaction involving metal magnesium and ethyl bromide in tetrahydrofuran to prepare ethyl magnesium bromide is carried out at reduced temperature, which maintains ethyl magnesium bromide in dissolved state, and is then reacted with acetylene gas under pressurizing condition, so that it immediately reacts with magnesium bromide group. Further, the reaction temperature that is near normal temperature, avoids ethyl magnesium bromide crystallization at low temperature, and at the same time byproduct ethane gas is enriched and is collected by light gas collecting device and discharged. The absorption of acetylene in tetrahydrofuran and ethyl magnesium bromide results in homogeneous reaction mixture that makes the reaction more complete, and improves purity of mifepristone intermediate. The method completely avoids excessive consumption of acetylene gas, reduces production cost, is economic, is environment friendly, and has good industrial application prospects.    DETAILED DESCRIPTION - Preparation of modified mifepristone intermediate involves: taking magnesium and ethyl bromide as raw materials, reacting at 25-39 degrees C in presence of tetrahydrofuran to obtain ethyl magnesium bromide; inletting acetylene gas in ethyl magnesium bromide, reacting to obtain propynyl magnesium bromide; adding 3-alpha,3-beta-ethylene-dioxide-5(10),9(11)-diene-17-one in propynyl magnesium bromide, reacting to obtain 3-alpha,3-beta-ethylene-dioxide-17-beta-hydroxy-(1-propynyl)-5(10),9(11)-diene; and collecting the compound. The mass ratio of magnesium sheet and ethyl bromide is 3:10, mass ratio of magnesium sheet and tetrahydrofuran is 1:6, and volume ratio of ethyl bromide and tetrahydrofuran is 3:10. In step of reacting acetylene gas in ethyl magnesium bromide reaction temperature is 15-60 degrees C, and reaction pressure is 0.01-0.3 MPa. The method further involves collecting and discharging the light gas (preferably ethane) produced in reacting acetylene gas in ethyl magnesium bromide. The mass volume ratio of 3-alpha,3 beta-ethylene-dioxide-17-beta-hydroxy-(1-propynyl)-5(10),9(11)-diene and tetrahydrofuran is 1:6.